Publish API docs

[bact]

Very basic API doc, using all pdoc defaults.

Version from "main" will be published to https://spdx.github.io/spdx-python-model/dev/.

Each tag-released version will be published to https://bact.github.io/spdx-python-model/<version_number>. (Only at the release time, no retrospec).

Note that pdoc does not support multiple versions natively. No drop-down menu for versions. If that is needed, may need to use Sphinx instead.

See demo at https://bact.github.io/spdx-python-model/dev/

[bact]

Comparing Sphinx (default theme & PyData theme which supports API versioning) and pdoc (default theme), pdoc navigation on the left pane is much easier to use.

• Sphinx lists all the classes in the content area, which maybe fine if number of classes is small, but in our case it's a lot.
• pdoc lists classes separately on the left pane, make it easier to navigate. The way it displays method signature also more compact.

Sphinx:

pdoc: