Docs: Refactor translation howto

The old howto included out-dated info on how to translate Oscar. That
has been replaced by a link to the Transifex project.
It was the only howto left under the Contributing headline, which really
shouldn't be there (as there's an entirely different section in the docs
for contributing). As it is more of a topic anyway, and will hopefully
soon include documentation on translation style etc., it was turned into
a topic.
Docs: Customisation topic to tie-in how-tos

Attempt at making it clearer what needs to be done for what kind of
customisation. Created a new topic that lines out the prepatory
steps that need to be taken, and moved duplicate information from the
how-tos into the new topic.
Docs: Fix broken sandbox link
Docs: Remove empty how_to_model_your_catalogue

It's a great idea, but confusing when empty. Replaced by links to
how_to_customise_models which contains some explanations.
Docs: Merge changes from #640 into new structure
Merge pull request #567 from tangentlabs/issue/567/docs

Improve docs on customising Oscar