Add Nitrokey Python SDK documentation

[robin-nitrokey]

Fixes: #329

[robin-nitrokey]

There are three options for fetching the SDK documentation:

1. Use a submodule (current implementation). Advantages: reproducible builds. Disadvantages: higher maintenance, uses submodules.
2. Use and download a pinned tag. Advantages: reproducible builds. Disadvantages: higher maintenance.
3. Always use the latest tag from GitHub. Advantages: reduced maintenance. Disadvantages: SDK releases can break the docs build.

I would vote for 1 or 2 to keep the build reproducible. 2. would be a manual way of re-implementing submodules, so my preferred option is 1.

[daringer]

can we please not have a submodule - download or checkout a tag?

[robin-nitrokey]

Updated to download the SDK from a tag. We need to maintain a symlink to the downloaded SDK, but I did not find a way to avoid that.

[daringer]

can you please rebase - then we should get a preview

[github-actions]

The successful documentation build was deployed for preview:

• https://docspreview.nitrokey.com/PR-334

[robin-nitrokey]

Would be good to also have the version displayed somewhere but I don’t see an option to automatically include it.

[robin-nitrokey]

Looks like we could read out the version in conf.py using Python code and then define a RST directive to insert the version string.