add reference docs for all command line arguments and env vars

[dirk-thomas]

Add the following documentation pages:

• [x] build verb
• [x] graph verb
• [x] info verb
• [x] list verb
• [x] metadata verb
• [x] mixin verb
• [x] test verb
• [x] test-result verb
• [x] Global arguments and environment variables
• [x] Executor arguments
• [x] Event handler arguments
• [x] Discovery arguments
• [x] Package selection arguments
• [x] Mixin arguments

Not covered verbs:

• extension-points verb (only relevant for debugging / development)
• extensions verb (only relevant for debugging / development)
• version-check verb (has no arguments)

[dirk-thomas]

A preview is available at: https://external-builds.readthedocs.io/html/colcon/57/index.html

[dirk-thomas]

Thanks for the review and the numerous fixes / improvements. I will go ahead and merge this big chunk. If you have any further comments I am happy to address them after the fact.