Merge pull request #17535 from sennyeya/api-docs-new-backend

[PRFC] API Docs for the whole backend

.changeset/gentle-pears-camp.md

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+---
+'@backstage/plugin-catalog-backend-module-backstage-openapi': minor
+---
+
+Adds a new catalog module for ingesting Backstage plugin OpenAPI specs into the catalog for display as an API entity.

.changeset/thirty-stingrays-grin.md

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+---
+'@backstage/backend-openapi-utils': minor
+---
+
+Adds a new route, `/openapi.json` to validated routers for displaying their full OpenAPI spec in a standard endpoint.