Api documentation

[LecrisUT]

I am planning to use the fmf framework for an academic project of mine, but I find the documentation should be improved:

• type-hints for the api classes
• equivalent api for the cli commands
• document how to read, access the data from an fmf tree
• document how to write an fmf tree
• document how to define/use Contexts
• document the source of the Context data (from the tmt tree and from built-ins)
• document extension apis. Currently there are none afaik, but there could be the plans for them. E.g. compatibility and extensions with linters/json-schema, or apis to define different abstract sources like archived sources