In odoo/documentation-user#945, the developer documentation is merged
with the user documentation in a single repository and build config.

This commit then removes source files for the developer documentation
from the odoo/odoo repository.

DOC PR: https://github.com/odoo/documentation-user/pull/945

task-2351938
task-2352371
task-2205684
task-2352544

Co-authored-by: Victor Feyens <vfe@odoo.com>
Co-authored-by: Antoine Vandevenne <anv@odoo.com>

This commit replaces calls to pycompat helpers that were intended for
python 2 <-> python 3 interoperability for python 3 builtins, as python
2 is no longer officially supported by Odoo.

This includes:
    * calls to imap/izip/ifilter replaced by map/zip/filter
    * uses of text_type replaced by str
    * uses of unichr replaced by chr
    * calls to implements_to_string, implements_iterator removed
    * string_types and integer_types replaced by str, int respectively
    * calls to to_native replaced by calls to to_text

This is done in preparation to the removal of these deprecated helpers
in the following commit.

* remove references to basestring & unicode (use relevant pycompat
  helpers)
* remove some str calls (either entirely or replaced by relevant
  helper, either text or native)
* use better API to avoid unnecessary conversions
* remove some XML declarations in views

In Python 3, all of these were "consolidated" under urllib(.request,
.parse, .errors) which is inconvenient.

Since we already have hard dependencies on requests and
werkzeug(.urls, which is a backport of Python 3's unicode-aware
urllib.parse) migrate *everything* to that.

A sticking point is urllib2.URLError, those were (mostly) replaced by
the slightly more general IOError which URLError extends.

Pretty much completely rewritten theme with custom HTML translator and a
few parts of the old theme extracted to their own extensions.

Banner images thought not to be that huge after all, and not worth the
hassle of them living in a different repository.

co-authored with @stefanorigano