Create a documentation page for each of the submodules of i18n

The submodules are:
- block translation
- field translation
- Multilingual select
- path translation
- string translation
- synchronize translations
- Multilingual content
- multilingual forum
- contact translation
- translation redirect
- translation sets

I've added all subpages except:
- multilingual forum
- contact translation
- translation redirect
- translation sets

I've marked some as incomplete and some as 'Needs Technical Review'. So far, I didn't touch the Multilingual Taxonomy or Menu Translation pages, which need some attention as well.

While I think having some page to document a list of submodules with a better description of what each of them provides is fine, I'm not that sure we should document the functionality module per module.

I'd rather have pages like 'Translate your taxonomy' 1. Enable the module 'Taxonomy translation'... (which btw is not on the list)

I think for user documentation we should focus on functionality and features better than modules. Modules come up eventually when we tell the user which modules are needed for this and that feature.

While for the taxonomy case the mapping is quite straight there are some others that are not like i.e. 'Translate labels and descriptions' (fields, menus, i18n_node, etc...)

The 'Menu Translation' and 'Multilingual Taxonomy' pages were already there, but do need updating. I'll think about how to reorganize these pages to make them more feature-focused rather than module-focused. Maybe this could involve renaming 'Submodules' to 'Features' and then rewriting the less feature-driven submodule pages like 'Multilingual Select', 'Multilingual Content' and 'String Translation'.

Ok but in any case I don't think we need the Modules/Features grouping. This doesn't allow people to see what's available at first sight.

OK, at this point in time only translation sets is missing from pixelite's list in #3

This is for Drupal 7 docs and Drupal 7 is out of date. So, this is now a won't fix.