Refs #32412 -- Adjusted Contributing Guide start page.

* Added headers emphasising Work on the framework vs Join the community
  sections, to raise the visibility of the community section.
* Added callouts to three main code/docs/translations areas, linking to
  respective start pages.
* Moved some Writing code specific content from the start page to the
  Writing code index. This clarifies the start page and adds content to the
  previously empty Writing code index.

Co-authored-by: Carlton Gibson <carlton.gibson@noumenal.es>

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