DFSG-incompatible : GFDL with a front-cover and a back-cover.

This is a temporary workaround that might help avoid issues like
https://todo.sr.ht/~pkal/compat/7 and perhaps
https://todo.sr.ht/~pkal/compat/5 too.

This will be reverted before the next release.

It appears that having an outdated compat-macs.el loaded can cause
issues when updating between versions with new macros.

According to the commonmark spec [0], this is allowed.  I prefer it
because it makes reading the labels and URLs easier, especially when a
block has multiple labels of varying lengths where tabulating wouldn't
make much sense.

[0] https://spec.commonmark.org/0.30/#link-reference-definition

The compatibility function assumes that we are using a version of
Emacs that does not support native compilation (hence the
implementation of 'subr-primitive-p' is just 'subrp').

This addresses https://todo.sr.ht/~pkal/compat/6.

To have to change also affect ELPA packages, we will remove
.dir-locals.el from .elpaignore.

This is a further simplification that avoids difficult edge cases.
Among other things this also satisfied Flymake, that copies and
renames files to a temporary directory, breaking the version inference
process.

That way we avoid accidentally autoloading rx.

Copy documentation from the Emacs Lisp manual and write our own where
missing.  Use proper Texinfo conventions instead of what the
ox-texinfo generated.  Also add subsections in each support chapter to
easily distinguish between default, prefixed and missing support.

As mentioned in NEWS.27:

    ** 'file-name-absolute-p' no longer considers "~foo" to be an absolute
    file name if there is no user named "foo".

This reverts commit fe923d8e.

Texinfo adds these anyway, so there is no need to mention it in the
link name.

As the default behaviour is just to load missing (unprefixed)
definitions, which are all compiled away if NM ≤ emacs-major-version,
there should be no functional difference.  The advantage is that fewer
files have to be opened, parsed and loaded.

Suggested by Daniel Mendler in this thread:
https://lists.sr.ht/~pkal/compat-devel/%3C87cze74icg.fsf_-_%40posteo.net%3E