Move to markdown

[dyspose]

Move the docs to markdown (probably mkdocs material). And merge language branches into a single one.

In simple: Ge at rid of RTFD.

If @Vladislav4KZ isn't aginst, of course.

[Vladislav4KZ]

What are the advantages that will come from this?

Read the Docs allows you to download documentation for offline reading in htmlzip-archive, pdf and epub formats.

Also it supports both reStructuredText and Markdown (but currently reStructuredText is used).

I once thought that maybe I should make the localization separate from the documentation files, so as not to duplicate images, configuration files, etc. in another branch.

Although some images are also different depending on the documentation language (images with text), and they can't seem to be replaced via Gettext (i.e. via localization files with .po extension)

And now I don't have time to do it, so I'm updating/editing what I have.