bug#73425: 31.0.50; Support images in HTML versions of ELPA package manuals

[Eshel Yaron via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Hi,

I'm opening this feature request following a discussion with Philip
elsewhere: Texinfo manual can include images, but AFAIK (and please
correct me if I'm wrong here) there's currently no way for ELPA packages
to include images in their manuals and have these images appear also in
the HTML version of the manual that the ELPA server builds and serves.

An example is my GNU ELPA package Kubed.  In the development version,
the manual refers to images which are present on my web server, where I
provide HTML versions[1][2] of the manual.  There, the images are
displayed properly.  On the ELPA server[3], OTOH, the images are nowhere
to be found, and we instead get just the alt text and caption.  That's
expected, but the alt text is shown with no distinctive styling so the
result seems quite confusing.

Ideally, I would like a way to be able to do one of the following:

- Include images in the package repository, somehow indicate in the
  package specification that the manual refers to these images, and have
  the ELPA server keep the images around in a predictable location, so I
  can link to it from the manual.
- Provide a IMAGE_LINK_PREFIX value (e.g. pointing to my web server)
  that the ELPA server will use while building the HTML manual.
- Override the creation of the HTML manual entirely, and have the ELPA
  server redirect or link to the version hosted on my web server.


Thanks,

Eshel

[1] https://eshelyaron.com/kubed.html
[2] https://eshelyaron.com/man/kubed/
[3] https://elpa.gnu.org/devel/doc/kubed.html