Add class links to top-level docs

[Former user]

Example Before: 
    ``Connection``

Example After: 
    :class:`~sqlalchemy.engine.base.Connection`

Limit scope to top-level docs (for the first pass):

diana@diana-laptop:~/workspace/sqlalchemy/doc/build$ ls -1 *.rst
copyright.rst
dbengine.rst
examples.rst
index.rst
intro.rst
mappers.rst
metadata.rst
ormtutorial.rst
session.rst
sqlexpression.rst

[Former user]

from [e-mail]

[Former user]

give me a few minutes... there's a typo in that first diff

[Former user]

do not use the first diff

[Michael Trier]

Fixed in 81410467f4c5f1dbecc5c3d5f0f183c56da2a4dc. Thank you Diana Clarke.

[Mike Bayer]

Removing milestone: 0.6.0 (automated comment)