docs: improve clarity of dotenv setup

[ethan-sargent]

Clarifies documentation by linking to dotenvy.
Removes the superscript note recommending the use of dotenvy as it is now the
linked repository.
This also ensures users are linked to maintained documentation
and avoids confusion as in allan2/dotenvy#92.

depends on allan2/dotenvy#98
fixes #3168
resolves root cause of allan2/dotenvy#92

[abonander]

You probably want to get this mention in the macro docs as well: https://github.com/launchbadge/sqlx/blob/main/src/macros/mod.rs#L56

[ethan-sargent]

Thanks @abonander - done. Looked for any other references but didn't spot any.

Updated the footnote names (neat trick!) to dotenvy for consistency as well.

edit: fixed an issue with the commit message – let me know if you want them squashed