dotenv files: State of the Art in DDEV

[rfay]

@stasadev has extended the capabilities of dotenv in DDEV quite a lot, and there is great tooling. Much of it is focused around add-ons, but it needn't be.

• Explain how .ddev/.env.* works and where it can be used
• Explain ddev dotenv set etc.
• Explain future possibilities for improvement
• Reiterate traditional techniques as well, https://ddev.readthedocs.io/en/stable/users/extend/customization-extendibility/

This blog post probably could be integrated with some changes to the docs as well, as our docs seem to be oriented toward the more traditional pre-dotenv techniques.