Added a new ccache doc to "Kerberos V5 concepts"

This is to add a short introductory document on credential
caches to the Concepts section of Kerberos documentation.

(cherry picked from commit 251f94621328277044e3ae1a10ecd3ddfdda1dbb)

https://github.com/krb5/krb5/commit/fa4a84050e9a8e11284096997e490e2fb811e78c
Author: Zhanna Tsitkov <tsitkova@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: fa4a84050e9a8e11284096997e490e2fb811e78c
Branch: krb5-1.12
 doc/basic/ccache_def.rst |  134 ++++++++++++++++++++++++++++++++++++++++++++++
 doc/basic/index.rst      |    1 +
 2 files changed, 135 insertions(+), 0 deletions(-)