pkg: documentation of `dune-workspace` and `lock_dir`

[Sudha247]

Dune package management by default uses upstream and overlays repositories to pull in packages. One can include custom repositories in the dune-workspace that is documented here: https://dune.readthedocs.io/en/latest/tutorials/dune-package-management/repos.html. What is unclear is how this configuration applies to dev tools. @Alizter pointed out in an offline discussion that, for adding constraints to dev tools, one has to mention it in the dune-workspace also by mentioning the path of the lock directory. Which would look something like.

(lock_dir
 (path "dev-tools.locks/ocaml-lsp-server")
 (constraints
   (ocaml-lsp-server (= 1.19.0+ox))
   (ocamlformat (= 0.26.2+ox))
   (ocaml-variants (= 5.2.0+ox)))
 (repositories overlay oxcaml upstream))

It will be good to document this in the tutorial or elsewhere. This could be affected by #12671, however we might need to keep this up to date as it's not clear from the docs currently how to achieve this. Also related to #12732.

[Alizter]

Note that you don't need to remember to write upstream and overlay, you can instead write :standard.

[Leonidas-from-XIV]

The tutorial is not the right place for it because in a tutorial you shouldn't need to change your dev-tool configuration (if you do then we did a bad job and need to fix it), but we could certainly have an explainer document on how dev-tools can be configured (at least with the current state).