Document using bundle-modules = [ "go" ] for Go packages that don't vendor dependencies

[jpmcb]

What I'd like:

There currently isn't any good documentation on using bundle-modules = ["go"] for packages that are pulled in and don't vendor their own dependencies (like hotdog).

We should write up a snippet on using this system on the buildsys markdown so that package authors can reference it when building in new packages.

Any alternatives you've considered:

There is some documentation on this directly in the buildsys code:

bottlerocket/tools/buildsys/src/manifest.rs

Lines 36 to 39 in 4b8a132

	The `bundle-*` keys on `external-files` are a group of optional modifiers
	and are used to untar an upstream external file archive, vendor any dependent
	code, and produce an additional archive with those dependencies.
	Only `bundle-modules` is required when bundling an archive's dependences.

which was added as part of the initial feature add: #2378

But this should really be in the markdown docs itself since we can't expect package authors to read the buildsys code.