Docs: Replace `#NUMBER` Trac issue references with `trac-NUMBER`

Docs: Replace `#NUMBER` Trac issue references with `trac-NUMBER`

This is a version of gh-4993 for the `3.x-stable` branch.

The GitHub UI treats `#NUMBER` as referring to its own issues which is confusing
when in jQuery source it's usually referring to the old deprecated Trac instance
at https://bugs.jquery.com. This change replaces all such Trac references with
`trac-NUMBER`.

A few of the references came with the Sizzle integration and referred to the
Sizzle GitHub bug tracker. Those have been replaced with full links instead.

A new entry describing issue reference conventions has been added to README.

Closes gh-4994
Ref gh-4993
Ref 5d5ea015114092c157311c4948f7cc3d8c8e7f8a