feat(v2): add ability to set custom heading id (#4222)

* feat(v2): add ability to set custom heading id

* Add cli command

* Fix slugger

* write-heading-ids doc + add in commands/templates

* refactor + add tests for writeHeadingIds

* polish writeHeadingIds

* polish writeHeadingIds

* remove i18n goals todo section as the  remaining items are quite abstract/useless

* fix edge case with 2 md links in heading

* extract parseMarkdownHeadingId helper function

* refactor using the shared parseMarkdownHeadingId utility fn

* change logic of edge case

* Handle edge case

* Document explicit ids feature

Co-authored-by: slorber <lorber.sebastien@gmail.com>

website/docs/guides/docs/docs-create-doc.mdx

@@ -44,6 +44,10 @@ The headers are well-spaced so that the hierarchy is clear.
 - that you want your users to remember
   - and you may nest them
     - multiple times
+
+### Custom id headers {#custom-id}
+
+With `{#custom-id}` syntax you can set your own header id.
 ```
 
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