chpldoc and mason doc improvements to wire project info

[e-kayrakli]

This PR makes several improvements for mason doc.

• The template sphinx configuration has couple of important fields programmable via new environment variables like CHPLDOC_PROJECT_NAME
• chpldoc now has the ability to use an existing sphinx project: a Mason package should be able to have its own conf.py + index.rst. Currently, chpldoc simply overrides it. This is exposed with --has-sphinx flag, which prevents coping the generic Chapel template.
• mason doc passes project name into the environment variable I mentioned above.

I will probably continue on this as I find time to improve the integration a bit better.

TODO:

• add a field to Mason.toml to pick up an existing sphinx config dir
• make Mason generate a stub of that config dir