Rewriting many things, and some fixes

I dedicated my last few days to rewriting many parts of the docs and this is the result.

Tbh `31 changed files with 1,338 additions and 818 deletions` is quite much so I'll overview the changes I made.

**Style stuff**
- Hotkeys were changed to use the `:kbd:` markup
- File paths were changed to use the `:file:` markup
- ``` ``.extension`` ``` stuff has been changed to use `*.extension*` everywhere. I am not too okay with this change (can be reverted/adjusted to the inline code again) but I wanted the docs to be consistent in regard to this. Tell me your opinion.

**Content changes**
- snippets and completions docs have been merged (extensibility + reference => extensibility only) because they mostly contained the same stuff
- Added settings and build system docs to the projects section
- Added many commands
- Added documentation on missing key API functions
- Added `"tabtrigger\tdescription"` docs
- Added contributing section to "about" page
- Updated copyright to 2013
... and probably many more

**Things I'd like to do/have done/see soonish**
- Package Control page. PC is a requirement for ST and the docs should indicate that, as well as lead the way to installing packages from github or bitbucket. Many packages could then just link to the doc page instead of repeating installation instructions again and again.
- Syntax Preferences.
- An actual README for this repo (I'd rather not do this on my own tbh)
- Consistent section line-up. I tried to use =h=, h=, h*, h-, h^ for parts I edited but I didn't apply this on all sections. Should be covered by the readme.
- Use `:menuselection:\`Start --> Programs\` ` for menus instead of `**Start | Programs**`.

Any feedback welcome because I pretty much modified many things freely as I saw fit.

FYI, I will now start with syntax definition docs with YAML files in a new branch which I'll base on this one.

(Btw, you're not on IRC, are you?)