Overhaul rendering of the manual

[Kodiologist]

• Closes Stop using Read the Docs #2417

Here are various changes, in combination with a commit I just pushed to hyhomepage, to clean up the manual and put it on hylang.org. You can see the result at http://hylang.org/hy/doc/doc-testing. URLs that now look like

https://docs.hylang.org/en/stable/syntax.html

will instead be

https://hylang.org/hy/doc/v0.26.0/syntax

• I use version numbers insted of stable and master because cool URIs don't change. We'll no longer provide official web versions of pre-release versions of the Hy manual, and that should no longer be terribly necessary now that Hy isn't breaking backwards compatibility so frequently. I'll redirect docs.hylang.org to the latest release.
• The file extension (.html) is gone because it's an implementation detail.
• en is gone because full translations, if they're ever written (which I doubt), will probably be hosted under different URLs, as with Python itself.

If there are no objections, I'll make similar arrangements for Hyrule's manual, too.

[goyalyashpal]

• I use version numbers insted of stable and master because cool URIs don't change.

but stable and master do help others to link to the latest versions of a documentation.

the expected is for the stable to automatically redirect to the link with latest version number

• en is gone because full translations, if they're ever written (which I doubt), will probably be hosted under different URLs, as with Python itself.

oh lol, on point :)