Copy our wiki doc under docs/ for GitHub Pages

[thilp]

• Copy all documentation from the wiki in a docs/ directory at the root of Transformer's repository.
• Add some static site generator there as well: I have chosen Sphinx.
• Add make doc to build the documentation locally.
• Migrate README.md, CHANGELOG.md, CONTRIBUTING.md, and CONTRIBUTORS.md to the reST format for homogeneity and inclusion in the Sphinx-generated documentation.
• Add documentation about:
  • Transformer's release process;
  • how to contribute: conventions, maintaining the documentation;
  • Transformer's plugin system (with pictures!).
• Add technical documentation (reST docstrings included in the Sphinx-generated HTML).

Part of #28.

Types of Changes

What types of changes does your code introduce? Keep the ones that apply:

• Documentation / non-code

Tasks

List of tasks you will do to complete the PR:

• Copy the wiki contents in docs/.
• Add a static site generator. Check that it works locally with the copied documentation.

Deployment Notes

Once this PR is merged:

• activate Read The Docs for this repository,
• in README.rst, replace the documentation link (currently pointing to the wiki) accordingly,
• deactivate the wiki tab.

[thilp]

Reviewers: You can build the documentation locally with make doc (sphinx is installed by Poetry so I recommend a make configure first), then open it with firefox/chrome/whatever docs/_build/html/index.html.

[thilp]

Alternatively, here is a zip file containing the generated HTML files: transformer-html.zip