Move hacking guide to root directory

The hacking guide is in the root directory of every project as
HACKING.rst, except for hacking itself.  Keep the hacking guide as the
index to the docs, but move the file to the root directory.

Change-Id: If128dfc004db2b7427f987ea8a9f643b6c653ce8
Cleanup HACKING.rst

* remove two rules covered by pep8 since we already say 'see pep8' and
  gate on them via the pep8 tool:
* use code notation where appropriate
* fix formatting around apache2 header rule
* update import exceptions block, to match DEFAULT_IMPORT_EXCEPTIONS

Change-Id: I3febff29694865948c309340a20820c020e43d15
Re-Add section on assertRaises(Exception

When hacking was originally part of nova, there was a section on
assertRaises but this was moved out of HACKING, and into
nova/tests/README.rst in nova change
I0e660b6c2a77c5613c5a523ba0a64c3d7f6dd128

Since hacking has been enforcing this rule, H202, from the beginning,
add this section back into HACKING.rst

Change-Id: Ia7e0c3f908136c75333cb66ff90c7990c80027be
Turn Python3 section into a list

This is in preparation for including rule numbers in HACKING.rst, also a
list is easier to read, to skim.

Change-Id: Icaf158730af5d17477e84c6e12c18ec8da81216e
Add Python3 deprecated assert* to HACKING.rst

When we added H234 and H235 to check for deprecated asserts in Python
3.x (assert(Not)Equals, assert_), we forgot to add them to HACKING.rst

Change-Id: Id91977d76935998f5e5ae19dc089ee46d013aacb