test: Add Emacs 27.1 in CI & Bump the minimal-required version of Emacs (24.5 for now)

* Document (in README.md) the proposed policy: supporting Emacsen from
  the versions packaged in Debian Stable / Ubuntu LTS until their EOS.

Uses of coq-callcoq need to correctly handle nil as result for
the case that coq-callcoq fails.

Additionally, add a regression test.

Fixes #551

This action checks that
- make -C doc magic works
- the manual is currently up-to-date
The second check fails when somebody changes variable or function
documentation of something that appears in one of the manuals
without updating the manuals at the same time. Further, it fails
when emacs changes such that the function `texi-docstring-magic'
produces different output.

extend the github workflow to additionally run the tests in
ci/compile-tests

href: https://github.com/coq-community/docker-coq/wiki#supported-tags

* Only build the info manual.

* Otherwise we would get the following issue:

$ texi2pdf PG-adapting.texi
You don't have a working TeX binary (tex) installed anywhere in
your PATH, and texi2dvi cannot proceed without one.  If you want to use
this script, you'll need to install TeX (if you don't have it) or change
your PATH or TEX environment variable (if you do).  See the --help
output for more details.

For information about obtaining TeX, please see http://tug.org/texlive,
Makefile.doc:50: recipe for target 'PG-adapting.pdf' failed
or do a web search for TeX and your operating system or distro.

On Debian you can install a working TeX system with
  apt-get install texlive

* This should solve the "(buffer-read-only #<buffer test1.v>)" error,
  probably due to permission issues.