Docs: Add contributing page to the docs

[jdillard]

What

• Add the Contributing page to the Sphinx docs, while keeping a single source for contributor guidance in the repository root.
  • The markdown was just converted to reST, no content changes should have been made.

Why

Contributor setup (including pre-commit) was easy to miss when it lived only in a root markdown file and was not part of the published documentation. One RST file satisfies both GitHub’s contributing file convention and the Sphinx site.

Preview

• https://sphinx-simplepdf--149.org.readthedocs.build/en/149/contributing.html

[patdhlk]

thanks for the PR