packages,plugins: add package documentation

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

packages/techdocs-common/api-report.md

@@ -321,6 +321,4 @@ export class UrlPreparer implements PreparerBase {
 // src/stages/prepare/types.d.ts:19:8 - (tsdoc-param-tag-with-invalid-name) The @param block should be followed by a valid parameter name: The identifier cannot non-word characters
 // src/stages/prepare/types.d.ts:21:33 - (tsdoc-escape-right-brace) The "}" character should be escaped using a backslash to avoid confusion with a TSDoc inline tag
 // src/stages/prepare/types.d.ts:21:16 - (tsdoc-malformed-inline-tag) Expecting a TSDoc tag starting with "{@"
-
-// (No @packageDocumentation comment for this package)
 ```

packages/techdocs-common/src/index.ts

@@ -13,6 +13,13 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
+/**
+ * Common functionalities for TechDocs, to be shared between techdocs-backend plugin and techdocs-cli
+ *
+ * @packageDocumentation
+ */
+
 export * from './stages';
 export * from './helpers';
 export * from './techdocsTypes';