GitHub apparently classifies `fish-shell` as a C project because we just
barely have more C than C++ due to vendoring pcre2. Update
.gitattributes for this. Also tell it about our documentation.
see https://github.com/github/linguist
Imported from https://github.com/zanchey/fish-build/
Work on #2020.
Note that this is explicitly removed from `git archive` tarballs in
keeping with the changes in 0effcf6df1b3ba2.