bug#62374: Missing @footnote support in texinfo module

Forwarded from <https://issues.guix.gnu.org/61964>.


Guile REPL snippet to reproduce:

--8<---------------cut here---------------start------------->8---
scheme@(guile-user)> (use-modules (texinfo))
scheme@(guile-user)> (use-modules (texinfo serialize))
scheme@(guile-user)> (display (stexi->texi (texi-fragment->stexi "@defun Foo\nLorem Ipsum.@end defun")))

@c %start of fragment

@anchor{defun-Foo}@defun Foo 
Lorem Ipsum.

@end defun


@c %end of fragment
scheme@(guile-user)> (display (stexi->texi (texi-fragment->stexi "@footnote{Foo}")))
ice-9/boot-9.scm:1685:16: In procedure raise-exception:
Throw to key `parser-error' with args `(#f "Unknown command" footnote)'.

Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
--8<---------------cut here---------------end--------------->8---


Regards,
Bruno