Add unmaintained `dotenv` & Co.

[pinkforest]

Closes #1254

@JohnTitor @est31 @8573 would you mind reviewing pls

Should we recommend dotenvy as an alternative or something else as per https://github.com/rustsec/advisory-db/issues/1254#issuecomment-1214313669 ?

Note: I would be slightly hesistant to proceed with unmaintained when the crate says it is intended to be used only in test / dev env - nonetheless there are associated monorepo dotenv_ crates which don't mention this and seem to be used elsewhere

Thx.