Documentation / UI clean up

As features have been added, things have been getting a little cluttered. Some things I would like to see done:

[x] Main README section trimmed down to a digestible summary of Zutilo features aimed at new users.
- It should include an overview of the kinds of functions present in Zotero along with links to the detailed documentation, the attachments discussion, the contributors page, and the changelog.
- [x] The installation instructions will likely remain in this main section. They should include also a getting started section mentioning the configurability of Zutilo's item menu functions and shortcuts.
[x] Detailed documentation page with sections for
- [x] Item menu functions
- [x] Firefox browser functions
- [x] Keyboard shortcuts (broken into subsections of similar functionality)
[x] Attachments discussion moved to separate page
[x] Changelog moved to separate page
[x] Contributors information moved to separate page
- [x] Remove all .html files from the git repository once this page exists and is updated to describe the build script that generates them.
[x] Set the default item menu function visibility on a per function basis. Right now, all functions default to the Zutilo submenu, but some of them are low use enough that they could start off hidden
[x] Assign categories to keyboard shortcuts and display them in the preferences tree view. Then have the tree sort by category, so that there is more organization rather than just one long list of shortcuts.
[x] Update translations for documentation and recent UI strings.

wshanks / Zutilo