Reorder template tag documentation.

[jonathanslenders]

Right now, the documentation for creation of custom template tags, using a (parser, token)-function, is mentioned above the creation of simple template tags.
It would be great to revert this order, and make clear that when possible a simple tag is recommended. (Not only for readability.)

Why?:
The ability of creating custom template tags using (parser, token) is great, but it's a lot harder or impossible to predict the behaviour or the change the template render engine if we ever have to in the future. Making sure that people are using as much as possible the built-in Node classes, would do for a nice abstract syntax tree in moste cases.

​https://docs.djangoproject.com/en/dev/howto/custom-template-tags/

See this ticket #20434 for related info.

[Aymeric Augustin]

That proposal makes sense to me.

[Horst Gutmann]

I definitely agree. We should encourage people to put far more logic into template tags and filters than in the templates themselves. Having the most complicated approach being mentioned first, doesn't help.

[Daniel Quinn]

​https://github.com/django/django/pull/1125

Basically I broke the whole tags section up into "Simple" and "Advanced", placing the simple stuff first.

[Tim Graham]

Existing pull request doesn't merge cleanly due to some recent commits.

[Daniel Roseman]

New pull request: ​https://github.com/django/django/pull/3522

[Julien Phalip <jphalip@…>]

In 524e71c9c20da57722b22b978d87690379ca4ade:

Fixed #20435 -- Reordered the custom template tags docs.

Introduced the various shortcuts before explaining the more complex
parser/render functionality.

Also removed non-decorator syntax: it's been years since Django
supported a Python version without decorators.