[tldoc] tl2021 manual translations

[Karl Berry]

I've done the basic updates for 2021 for the TL Guide (texlive-en.tex,
r57999).  No doubt you will find bugs and improvements as you update the
translations, as always; just let me know.

The GUI screenshots need updating, and probably also the Windows support
text. (Siep, when you have time, please ... ?)

The current sources are committed to svn. If you don't have an svn
checkout, you can download texlive-en.tex and tex-live.sty from
  http://tug.org/svn/texlive/trunk/Master/texmf-dist/doc/texlive/texlive-en/
Let me know if any difficulties getting the new stuff.

Also as usual, for your (w)diff pleasure, the previous year (2020)'s
files are in the archive/ subdir of
Master/texmf-dist/doc/texlive/texlive-en/ in the TL tree, aka
https://tug.org/svn/texlive/trunk/Master/texmf-dist/doc/texlive/texlive-en/archive/

I don't plan to make any changes to the top-level README this year, but
I believe the .po files will get some substantial updates in days to come.

Let me know if any questions or problems. --thanks, karl.