Documentation build preview bot

[ludgerpaehler]

(Vercel) bot to preview documentation builds is to be set up, and run whenever a PR is openened against the docs. If there are better solutions than Vercel for this task, they should be used.

[ludgerpaehler]

Bot activated by now, but erroring out so far.

[ludgerpaehler]

Present error:

WARNING: Failed to import hydrogym.
Possible hints:
* ValueError: not enough values to unpack (expected 2, got 0)
* KeyError: 'hydrogym'
WARNING: [autosummary] failed to import hydrogym.
Possible hints:
* ValueError: not enough values to unpack (expected 2, got 0)
* ValueError: not enough values to unpack (expected 2, got 1)
* KeyError: 'hydrogym'

Extension error (sphinx.ext.autosummary)!

Tried approaches:

• Changing root path of the project defined in conf.py
• Updating Sphinx' packages to the present standard
• More direct addressing of HydroGym's modules such as e.g. hydrogym.core