Update/Include LDAP

[svx]

Currently we have in various places the mention of: plone.app.ldap like in
develop/plone/sessions/login.rst, manage/deploying/performance/ramcache.rst, etc.

We still have no part, where we explain in an understandable way, how you connect your Plone site with LDAP exactly.

Questions:

• should we replace plone.app.ldap with pas.plugins.ldap ?
• in how far we do have to rewrite manage/deploying/performance/ramcache.rst if we do that ?