fix(v2): fix title logic (meta vs heading) + ignore fixed anchor id syntax (#4688)

* parseMarkdownContentTitle should ignore {#my-anchor-id} syntax

* use frontMatter.title in priority for page meta title

* parseMarkdownString should ignore fixed anchor ids syntax

* docs: make the distinction between headingTitle + metaTitle more clear + add useful todo

* docs: make the distinction between headingTitle + metaTitle more clear + add useful todo

* writeHeadingIds should ignore top-level md title like "# Title"
=> we are not supposed to create anchor links for h1 headers

* update tests

* fix doc tests

packages/docusaurus-plugin-content-docs/src/__tests__/index.test.ts

@@ -309,7 +309,7 @@ describe('simple website', () => {
         'foo',
         'bar.md',
       ),
-      title: 'Bar',
+      title: 'Remarkable',
       description: 'This is custom description',
       frontMatter: {
         description: 'This is custom description',