repo-wide: Unify spelling/format of "`.eopkg` file(s)"

[Staudey]

Description

This unifies how .eopkg files are called throughout the docs. Previously there were all sorts of formats: *eopkg, eopkg, .eopkg, *.eopkg

I've decided on .eopkg because my other preference *.eopkg was tautological in some cases where the wildcard was confusing. eopkg is easily confused with the application itself and *eopkg seem like a worse version of *.eopkg

Also format some mentions of the eopkg program correctly inside back-ticks, slightly correct the referenced name of a eopkg config file section, and some other minor spelling/formatting/grammar fixes found while searching the docs for places where .eopkg files are mentioned

[davidjharder]

This is great. Can you add a sentence about this to the style guide page too

[Staudey]

I've added an eopkg-specific section to the Style Guide. Not sure if we want to unify this for all files. I just saw some other file talk in e.g. the package.yml page, which includes the * wildcard (for example *.py), but in those isolated cases it wasn't confusing, unlike when I replaced all .eopkg references with *.eopkg

[ermo]

I like this.