Difficult (impossible?) to find default port used by vision service REST

[littlebee]

One thing I noticed is that the constants doc no longer includes the default values; for example, https://littlebee.github.io/basic_bot/Api%20Docs/commons/constants/#bb_vision_port

But even with that information, it would still be difficult to find which port the vision service uses without prior knowledge.

We may need a custom doc generator to capture the default values from commons/constants.py and move constants doc out to it's own top level doc....

While typing the sentence above, I remembered doing this a while ago, see https://littlebee.github.io/basic_bot/Configuration/Environment%20Variables/ I think maybe,

1. Need to drop some links around in doc comments on constants.py and vision.py, referencing the "Environment Variables" doc page.
2. Improve "Environment Variables" docs page. Right now, it just spits the constants.py file out whole. Maybe need to parse the file and extract the three pieces of data - Env var name, default value, and comment. The generator for that page should also sort and right index by var name.