Add help for generators

Goal: provide users with better website help for Loco generators.

Context: The Loco guide website has help files for models, views,
controllers, etc., but not (yet) for generators. In my opinion,
generators are awesome and are a good way for novices to create Loco
apps quickly and correctly.

loco-rs#1094

This commit is a rough draft of a Loco guide help page. Ideally other
people here can use this as a starting point for adding more help,
better help, good examples, etc.

For me personally, this page as-is (presuming there are no showstopper
errors) would be a big help for teaching Loco to teammates, even given
the page is a rough draft.

I tried to use a similar TOML front matter; please can someone on the
team could proof that?

The Rails guides have a much more sophisticated geneators guide. IMHO
worth a read. In my Rails work, my teams often use custom generators,
and always use custom field types defined in a migration helper class,
such as `email = string(254)` and `latitude = numeric(12,9)`.

with '#' will be ignored, and an empty message aborts the commit.  # #
Date:      Thu Dec 26 15:43:19 2024 -0800 # # On branch
add-help-for-generators # Changes to be committed: #	new file:
docs-site/content/docs/the-app/generators.md #