fix(markdown): Don't use MDX components to construct headers

Closes #4776

This is a somewhat low-tech fix for this problem.
That's because it's not entirely trivial to resolve this as the MDX
components rely on the React context to figure out the title of the
page, but the remark plugin runs in an SSR context and does not have
access to the React contenxt.

Futhermore, while we could load the context statically for dev (given
the navigation context for the docs is availabel in the prebuild data)
this would lead to problems for org because we don't know inside this
remark plugin what language the user requested.

And while I'm certain we could figure it out given enough effort, it
hardly seems worth is since this is used in very few places.

In addition, it would do nothing to solve the matter of the nested a
tags when putting a link in the title, which clashes with the
auto-generated amchor links for titles.

So new rules:

- Title cannot be a link
- You can not use `DocsTitle` as title

markdown/dev/contribute/en.md

@@ -4,13 +4,13 @@ title: Contribute
 
 Looking to contribute to FreeSewing? That's wonderful.
 
-## <DocsLink slug="howtos/ways-to-contribute" />
+## Ways to contribute
 
 There are many ways to contribute, here's some examples:
 
 <ReadMore root='howtos/ways-to-contribute' />
 
-## <DocsLink slug="guides/code-of-conduct" />
+## Code of Conduct
 
 All FreeSewing contributors must respect and uphold our Code of Conduct: