Recreate a shell of @backstage/techdocs-common for smooth deprecation process

Signed-off-by: Eric Peterson <ericpeterson@spotify.com>

.changeset/techdocs-byta-namnet.md

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+---
+'@backstage/plugin-techdocs-node': patch
+---
+
+Renamed `@backstage/techdocs-common` to `@backstage/plugin-techdocs-node`.

.changeset/techdocs-empty-office.md

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+---
+'@backstage/techdocs-common': patch
+---
+
+**DEPRECATION**
+
+The `@backstage/techdocs-common` package is being renamed `@backstage/plugin-techdocs-node`. We may continue to publish changes to `@backstage/techdocs-common` for a time, but will stop doing so in the near future. If you depend on this package, you should update your dependencies to point at the renamed package.

packages/techdocs-common/.eslintrc.js

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+module.exports = {
+  extends: [require.resolve('@backstage/cli/config/eslint.backend')],
+};

packages/techdocs-common/README.md

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+# @backstage/techdocs-common
+
+**WARNING**: This package is moving to `@backstage/plugin-techdocs-node`.
+Please update any dependencies you may have, as this package will no longer be
+published or updated in the near future.
+
+Common node.js functionalities for TechDocs, to be shared between techdocs-backend plugin and techdocs-cli
+
+This package is used by `techdocs-backend` to serve docs from different types of publishers (Google GCS, Local, etc.).
+It is also used to build docs and publish them to storage, by both `techdocs-backend` and `techdocs-cli`.
+
+## Usage
+
+Create a preparer instance from the [preparers available](/plugins/techdocs-node/src/stages/prepare) at which takes an Entity instance.
+Run the [docs generator](/plugins/techdocs-node/src/stages/generate) on the prepared directory.
+Publish the generated directory files to a [storage](/plugins/techdocs-node/src/stages/publish) of your choice.
+
+Example:
+
+```js
+async () => {
+  const preparedDir = await preparer.prepare(entity);
+
+  const parsedLocationAnnotation = getLocationForEntity(entity);
+  const { resultDir } = await generator.run({
+    directory: preparedDir,
+    dockerClient: dockerClient,
+    parsedLocationAnnotation,
+  });
+
+  await publisher.publish({
+    entity: entity,
+    directory: resultDir,
+  });
+};
+```
+
+## Features
+
+Currently the build process is split up in these three stages.
+
+- Preparers
+- Generators
+- Publishers
+
+Preparers read your entity data and creates a working directory with your documentation source code. For example if you have set your `backstage.io/techdocs-ref` to `url:https://github.com/backstage/backstage.git` it will clone that repository to a temp folder and pass that on to the generator.
+
+Generators takes the prepared source and runs the `techdocs-container` on it. It then passes on the output folder of that build to the publisher.
+
+Publishers gets a folder path from the generator and publish it to your storage solution. Read documentation to know more about configuring storage solutions.
+http://backstage.io/docs/features/techdocs/configuration
+
+Any of these can be extended. We want to extend our support to most of the storage providers (Publishers) and source code host providers (Preparers).

packages/techdocs-common/api-report.md

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+## API Report File for "@backstage/techdocs-common"
+
+> Do not edit this file. It is a report generated by [API Extractor](https://api-extractor.com/).
+
+```ts
+export * from '@backstage/plugin-techdocs-node';
+
+// (No @packageDocumentation comment for this package)
+```

packages/techdocs-common/package.json

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+{
+  "name": "@backstage/techdocs-common",
+  "description": "Common node.js functionalities for TechDocs, to be shared between techdocs-backend plugin and techdocs-cli",
+  "version": "0.11.11",
+  "main": "src/index.ts",
+  "types": "src/index.ts",
+  "private": false,
+  "publishConfig": {
+    "access": "public",
+    "main": "dist/index.cjs.js",
+    "types": "dist/index.d.ts"
+  },
+  "backstage": {
+    "role": "node-library"
+  },
+  "homepage": "https://backstage.io",
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/backstage/backstage",
+    "directory": "packages/techdocs-common"
+  },
+  "keywords": [
+    "techdocs",
+    "backstage"
+  ],
+  "license": "Apache-2.0",
+  "files": [
+    "dist"
+  ],
+  "scripts": {
+    "build": "backstage-cli package build",
+    "lint": "backstage-cli package lint",
+    "test": "backstage-cli package test",
+    "prepack": "backstage-cli package prepack",
+    "postpack": "backstage-cli package postpack",
+    "clean": "backstage-cli package clean",
+    "start": "backstage-cli package start"
+  },
+  "bugs": {
+    "url": "https://github.com/backstage/backstage/issues"
+  },
+  "dependencies": {
+    "@backstage/plugin-techdocs-node": "^0.11.11"
+  },
+  "devDependencies": {},
+  "jest": {
+    "roots": [
+      ".."
+    ]
+  }
+}

packages/techdocs-common/src/index.ts

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+/*
+ * Copyright 2020 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+export * from '@backstage/plugin-techdocs-node';