Make it easier to configure docsite builds, allow to override collection URLs and installation instructions

[felixfontein]

Depends on ansible-community/antsibull-core#7.

Fixes #15.

[felixfontein]

The PR now contains code that allows to override the way a collection URL is composed, and the way the collection installation command is composed. This resolves #15.

For example, you can write the following in the configuration file for a docsite (the sphinx-init command now creates a default one which has some explanations for these options):

collection_url = {
  * = "https://galaxy.ansible.com/{namespace}/{name}"
  community.crypto = https://ansible.fontein.de/collections/community/crypto/
  felixfontein.* = "https://galaxy.fontein.de/{namespace}/{name}"
}

collection_install = {
  * = "ansible-galaxy collection install {namespace}.{name}"
  community.* = "git clone https://github.com/ansible-collections/community.{name}.git ~/ansible/collections/ansible_collections/community/{name}"
}

(In case you're wondering, the configuration file format is perky's format: https://github.com/larryhastings/perky/#readme)

[felixfontein]

FYI: before this can be un-WIP-ed, ansible-community/antsibull-core#7 needs to be merged, antsibull-core 1.2.0 released, and f5a727d removed from this branch. But that's basically the only thing that's missing from my side.

[briantist]

this is great! really looking forward to it

[briantist]

excellent, this should make for some interesting possibilities in https://github.com/ansible-community/github-docs-build/ maybe 🤔

[felixfontein]

Now that antsibull-core 1.2.0 has been released, this can be merged.

[felixfontein]

@briantist thanks for reviewing this!