Consolidate RST install docs into larger files

Consolidate RST install docs into larger files

Create two larger files (install_kdc.rst and install_clients.rst) and
remove the install_kdc and install_clients directories.  No
significant changes to content.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25724 dc483132-0cff-0310-8789-dd5450dbe970