Add documentation for depexts (#13657)

[sabine]

Add a new reference page documenting external system dependencies (depexts):

• declaration in opam files,
• lock directory format (standard and portable),
• the dune show depexts command,
• platform variables with environment overrides, and
• build failure hints.

This is a first PR to lay the groundwork (reference docs) for #13657. There will be one or more followups to add the remaining new pages (tutorial, how to), and to make the addition to the existing doc.

[Leonidas-from-XIV]

Thanks for starting the documentation on depexts.

[sabine]

Thanks @Alizter for review comments, addressing this now

[Leonidas-from-XIV]

We discussed this at standup and I was asked to re-review it. So my conclusion is that to merge it still needs a bit of rework:

1. It needs a ticket to track that there should be a way to declare depexts without having to go through .opam.template files. Projects using dune pkg don't even need to generate .opam files usually so forcing these files onto the projects because dune-project does not support depexts is really unfortunate.
2. It is not a reference. It's an explanation or maybe a howto and thus should be moved accordingly. The reference part of this is very short or even non-existent because the feature we'd want to document (specifying the depexts of my project) doesn't exist (see my first point).
3. @sabine, you need to sign off the commits (to pass the DCO check) and possibly rebase. I don't think the CI failures are related to the documentation but it would still be good to have this PR pass CI successfully.

[sabine]

Ah sorry for dropping the ball on this.

• Rudi found an open issue that matches the feature request here: dune-project stanza for opam depext #5506.
• since the doc as such is very short, it seems like the right place to move it is after the "Building" section, before "Packaging for Dune Compatibility", so this is where it is now
• moved the "dune doesn't install or manage system dependencies" statement earlier in the explanation
• do we want to call out the limitation of needing opam template files when using depexts? (I put it just as a TODO comment in the doc, but we could make it a user-facing disclaimer until the feature request is completed.)
• added a How To on how to deal with depexts

I don't see anything right now that could be done to make this clearer, so I estimate this needs a final review (especially the new How To document)? I'm waiting for CI to finish to fix any remaining errors if any.

[Leonidas-from-XIV]

Thanks for picking it up again. Looks good overall, I only have a few small comments to clarify.

CI looks good now, the benchmarking failure can probably be disregarded.

[Leonidas-from-XIV]

Thanks! Undraft once you think it is ready to be merged.