feat: simplify base/page/form template structure; add docs

i'm sure there is more to document yet but this gets us started

docs/narr/templates/lookup.rst

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+
+Template Lookup
+===============
+
+The discovery of templates is handled by Mako, and is configurable.
+
+WuttaWeb comes with all templates it needs, in the path designated as
+``wuttaweb:templates``.
+
+When the app renders a page, it invokes the Mako lookup logic, which
+searches one or more folders and returns the first matching file it
+encounters.  By default ``wuttaweb:templates`` is the only place it
+looks.
+
+A template is searched for by "name" but it is more path-like, e.g.
+``/page.mako`` or ``/master/index.mako`` etc.  So for example the file
+at ``wuttaweb:templates/home.mako`` is used for home page (using
+lookup name ``/home.mako``) by default.
+
+
+.. _mako-template-override:
+
+Overriding the Search Paths
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The basic idea is to give it a list of paths it should search when
+trying to find a template.  The first template file found for a given
+search name is used and no further search is done for that name.
+
+You can define the Mako lookup sequence in your ``web.conf`` as
+follows:
+
+.. code-block:: ini
+
+   [app:main]
+   mako.directories =
+       /random/path/on/disk
+       poser.web:templates
+       wuttaweb:templates
+
+This setting is interpreted by ``pyramid_mako`` (`docs`_).
+
+.. _docs: https://docs.pylonsproject.org/projects/pyramid_mako/en/latest/index.html#mako-directories
+
+Here ``wuttaweb:templates/home.mako`` would still be used by default
+for home page, *unless* e.g. ``/random/path/on/disk/home.mako``
+existed in which case that would be used.
+
+Each path can have an arbitrary set of templates, they will
+effectively be combined to a single set by the app, with the
+definition order determining search priority.
+
+If you are already using a custom ``app.main()`` function for
+constructing the web app during startup, it may be a good idea to
+change the *default* search paths to include your package.
+
+Setup for custom ``app.main()`` is beyond the scope here, but assuming
+you *do* already have one, this is what it looks like::
+
+   from wuttaweb import app as base
+
+   def main(global_config, **settings):
+
+       # nb. set the *default* mako search paths; however config can
+       # still override with method shown above
+       settings.setdefault('mako.directories', ['poser.web:templates',
+                                                'wuttaweb:templates'])
+
+       return base.main(global_config, **settings)