Improve docs.txt

[vazhnov]

• Rename "docs.txt" into "docs.rst" to be recognisible as reStructuredText.
• Remove extra spaces.
• Fix typos.
• Improve reStructuredText formatting: inline code blocks, XML code block.
• \ wasn't rendered.
• Improve wording.
• Split to paragraphs to avoid too long text.
• Chapter "texturecache.py": highlight flags in a list
• Adjust headers level: "texturecache.py" and "Deep clean" should not be under "Settings"
• Chapter "Advanced usage": very confusing "NOTE", I think the last sentence should not be inside the NOTE.

Please see the rendered version here: https://github.com/vazhnov/Kodi_database_cleaner/blob/Improve_docs_rst/docs.rst.

I can split these changes into smaller merge requests, if needed. At least, I can split the file renaming into a dedicated merge request, so the diff will be more clear.

TODO: it would be nice to add screenshots. As another merge request 🙂