feat(cli): add post-build argument

[e-roux]

Context

It is currently possible to run pre-build commands. In some situation, it can be interesting to run commands after sphinx-build, e.g. tailwindcss.

Proposal

This PR adds support for post-build arguments copied on the existing pre-build.

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[e-roux]

Updated the PR to the latest source on main

[AA-Turner]

The implementation seems fine, but perhaps we could sketch a use-case or two in the README?

A