gopls/doc: add documentation for running gopls as a daemon
gopls/doc: add documentation for running gopls as a daemon

Some initial documentation is added describing how to use a shared gopls
daemon, following the work in golang.org/issues/34111.

Updates golang/go#34111

Change-Id: I8ee07d7415ae799bb867973c34bda53f9a35126d
Reviewed-on: https://go-review.googlesource.com/c/tools/+/221138
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools