Add support for generating meta tags.

[ztangent]

This PR addresses issue #1321 by adding support for generated meta tags.

As discussed in that issue, OpenGraph and Twitter meta-tags are generated to enable link previews. The following functionality is provided:

1. Ability to specify a preview image, that auto-populates the og:image and twitter:image meta tags.
   • This is done by creating preview.{png,webp,gif,jpg,jpeg} in the assets directory. Because meta tags need to link an absolute URL, the canonical URL for the image is constructed from the canonical option in the HTML constructor.
2. Auto-populate title, og:title and twitter:title with the same page_title value that gets used in the <title> tag.
3. Ability to specify the description, og:description and twitter:description meta tags with a string.
   • A site-wide description can be specified as a keyword argument to makedocs, and can be over-ridden in a page-specific manner using the Description keyword in an @meta-block.
4. Auto-populate the og:url and twitter:url metatags with the canonical URL of that page, as long as canonical is set.

Documentation has been updated accordingly. The only thing I haven't done is write test cases for this functionality, because I'm not quite sure where to add them or how to test for the existence of the correct meta-tags in some generated HTML file. Advice would be appreciated!

[oxinabox]

Can we fallback to the logo if the preview image isn't found?

[ztangent]

@mortenpi suggested not having a fallback because the preview doesn't look the best (see #1321 (comment)), but it'd be an easy change either way!

[mortenpi]

A few more things:

1. One last thing that would be nice is to have a couple of small occursin tests, mostly to make sure that the conditional behaviors are handled correctly.
2. It could use a CHANGELOG entry.

However, other than changing it to be an argument to HTML and the CHANGELOG, this looks great as is. Everything else is just bikeshedding.

[ztangent]

Made all of the suggested changes!

I can also add this test preview image to the real documentation for Documenter.jl if you'd like:

(*Note that even if I add it, it will only start showing up as a preview image with the next stable release of Documenter.jl, due to how the preview image URL is constructed from the canonical URL.)

[ztangent]

Have tested that this actually works with the documentation site for PDDL.jl: https://juliaplanners.github.io/PDDL.jl/dev/

You can enter the URL above into metatags.io to check, or just inspect the meta tags of the generated docs for dev.

[mortenpi]

LGTM.

I can also add this test preview image to the real documentation for Documenter.jl if you'd like:

I you don't mind, that would be awesome. We can merge as soon as that is done.

[ztangent]

Done!

[mortenpi]

Thanks!