Add a krb5-config man page

Missed when converting the old nroff man pages.

(cherry picked from commit 5a6d14918a3b5d003f248b66e88028f1b5e450b7)

https://github.com/krb5/krb5/commit/63599d8d0eb5bf82b1506cf81dcad27ce529d8b2
Author: Ben Kaduk <kaduk@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: 63599d8d0eb5bf82b1506cf81dcad27ce529d8b2
Branch: krb5-1.11
 doc/conf.py                            |    1 +
 doc/user/user_commands/index.rst       |    1 +
 doc/user/user_commands/krb5-config.rst |   85 ++++++++++++++++++++++++++++++++
 src/man/Makefile.in                    |    4 +-
 4 files changed, 89 insertions(+), 2 deletions(-)