Add project documentation

[ItsDrike]

The project should have an easily accessible and readable documentation, where it's explicitly state what is or isn't a part of the public API, and hence where the backwards compatibility will be ensured, including a few reference examples, and anything else that could be useful for the users, but also for contributors.

To keep track of everything from a single place, this issue should be linked in involved PRs, and below is the list of all features that need to be done before this issue can be marked as resolved:

• Improve project docstrings
  • Convert all docstrings to sphinx's (rst) format [DONE: Use sphinx rst format in all docstrings #18]
  • Mention sphinx docstring style in CONTRIBUTING.md [DONE: Update docstring formatting directive in CONTRIBUTING #286]
  • Add pycodestyle ruff linter extension, enforcing docstrings are present in all public functions [DONE: Enable pycodestyle ruff rules #179]
  • Look into enforcing that the docstrings are written in sphinx-compatible format?
• Project decisions that need to be made
  • Decide exactly which parts of the code-base should be a part of the public API, and which shouldn't [DONE: Move to mkdocs #346]
  • Pick a theme for the docs [DONE: Improve documentation style #28]
• Add sphinx dependency and create docs [DONE: Add sphinx #2]
• Create a readthedocs page for the project [DONE: here!]
  • Add badge linking to this documentation to README [DONE: Add sphinx #2]
  • Enable running documentation build CI workflows on each PR in readthedocs config [DONE: Add sphinx #2]
• Docs pages to be written
  • Contributing guidelines (currently in CONTRIBUTING.md file.) [DONE: Move to mkdocs #346]
    • Split things up, the current version is needlessly too long [DONE: Move to mkdocs #346]
    • Replace the content of the original CONTRIBUTING.md with a link to this docs page [DONE: Move to mkdocs #346]
  • Installation instructions (currently a part of README.md file.) [DONE: Move the Installation page from README to docs installation page #141]
  • Code of conduct (currently in CODE-OF-CONDUCT.md file.)
    • Replace content of the original CODE-OF-CONDUCT.md with a link to this docs page [DONE: Move Code of Conduct into docs #40]
  • Changelog page (just include CHANGELOG.md) [DONE: Add sphinx towncrier extension and link changelog #27]
  • Pages with examples, including the ones currently in README.md [DONE: Move to mkdocs #346]
  • Explanation of packet format, and some more detailed things about mc protocol? Linking https://wiki.vg might be enough though

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• Rewrite the docs using mkdocs, alongside fulfilling a bunch of other points: [DONE: Move to mkdocs #346]