Document use of tool.render-engine #29

Currently the only known tool that needs to look at the module/site from the pyproject.toml.

That said with a tui in development and with the potential for more tools to be added, there should be a singular place that lists the module and the site.

There should be some developer documentation that encourages the pattern

[tool.render-engine]

module = "<PATH.TO.MODULE>"
site = "<SITE.OBJECT>"

[tool.render-engine.] # tool.render-engine.cli/ tool.render-engine.tui/etc
<APP_SPECIFIC_COMMAND> = ""

It should be stated that projects should likely check both places for some values.

You could put module/site in the app specific but it makes sense if you're using multiple tools to put information that could be shared.