Import the GSS-API installation doc

There is an installation doc in the GSS-TSIG wiki (https://gitlab.isc.org/isc-external/kea-gss-tsig/-/wikis/Install): it should be integrated in Kea documentation (note the with-gssapi is included in the last release so this is required).

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information