Add output of --help in the documentation

[rgaiacs]

Pixi is having problems to install all the dependencies.

pixi run docs

returns

Error:   × failed to solve the pypi requirements of environment 'docs' for platform 'linux-64'
  ├─▶ failed to resolve pypi dependencies
  ╰─▶ Because empack>=5.1.1 depends on click<8.2 and click==8.3.0, we can conclude that empack>=5.1.1 cannot be used.
      And because only the following versions of empack are available:
          empack<=5.1.1
          empack==5.1.2
          empack==6.0.0
      and jupyterlite-xeus==4.2.2 depends on empack>=5.1.1, we can conclude that jupyterlite-xeus==4.2.2 cannot be used.
      And because only jupyterlite-xeus<=4.2.2 is available and repo2wasm==0.4.1.dev1+gf58c9dd28.d20251015 depends on jupyterlite-xeus>=4.2.2, we can conclude
      that repo2wasm==0.4.1.dev1+gf58c9dd28.d20251015 cannot be used.
      And because only repo2wasm==0.4.1.dev1+gf58c9dd28.d20251015 is available and you require repo2wasm, we can conclude that your requirements are
      unsatisfiable.
      
      hint: Pre-releases are available for `jupyterlite-xeus` in the requested range (e.g., 4.3.0a2), but pre-releases weren't enabled (try: `--prerelease=allow`)
  help: The following PyPI packages have been pinned by the conda solve, and this version may be causing a conflict:
        click==8.3.0