The covers the porting of content for `PackageCollections.md` into our
DocC catalog and fixes #8589.

The new guide article for `PackageCollections` exists under
`Sources/PackageManagerDocs/Documentation.docc/PackageCollections.md`
and touches on creating, publishing, signing/protecting, and modifying
package collections. Below the overview there is a list of subcommands
available on the `swift package-collection` tool with links navigating
to the Swift commands documentation.

Each of the `PackageCollection` CLI commands have been augmented with
supplementary information about how to use the subcommand (if
applicable), all of which is taken from the original
`/Documentation/PackageCollections.md` markdown file.

Some links within these guides either still reference the old links in
the original `Documentation/PackageCollections.md` file or are
placeholders - both are marked with a `TODO` to update these links once
other issues under #8603 are completed and we can replace the links with
the appropriate DocC equivalent.

Some content in these guides are also taken from
`Sources/PackageCollectionsModel/Formats/v1`.