[doc] Update copyright year to 2021 and update release manual

See merge request frama-c/frama-c!3452

[wp/doc] terminates is implemented

See merge request frama-c/frama-c!3437

24.0-beta release preparation

See merge request frama-c/frama-c!3435

- use https for URL
- fix URL of frama-c-discuss
- update quick install instructions for opam 2.1.0
- use --dev-repo for opam pin instructions

depext is now bundled in opam directly. Explicit installation instructions
are kept for reference.

Feature/andre/libc stdatomic

See merge request frama-c/frama-c!2203

[Kernel] add wkey for assembly warnings

See merge request frama-c/frama-c!3371

Changelog

See merge request frama-c/frama-c!3427

Resolve "[volatile] read const volatile variables"

See merge request frama-c/frama-c!2426

Terminate the support of terminates (almost)

See merge request frama-c/frama-c!3429

- use of cannot is apparently preferred over can not
- hov box without formatting hints is useless. Besides, opening
  an inner box very late on the line is usually not a good idea.