ci: validate "linkgit:" in documentation

ci: validate "linkgit:" in documentation

It is easy to add incorrect "linkgit:<page>[<section>]" references
to our documentation suite.  Catch these common classes of errors:

 * Referring to Documentation/<page>.txt that does not exist.

 * Referring to a <page> outside the Git suite.  In general, <page>
   must begin with "git".

 * Listing the manual <section> incorrectly.  The first line of the
   Documentation/<page>.txt must end with "(<section>)".

with a new script "ci/lint-gitlink", and drive it from "make check-docs".

Signed-off-by: Junio C Hamano <gitster@pobox.com>