Remove the 'Use Tails' categories and create new ones:

- First steps with Tails
- Connect to the Internet anonymously
- Encryption & privacy
- Work on sensitive documents
- Advanced topics

Each one corresponds to a topic specific intro page, eg. first_steps.mdwn, and a
category index, first_steps.index.mdwn, which is in turn inlined in doc.mdwn and
first_steps.mdwn to avoid duplication.

Conflicts:
	wiki/src/about.de.po
	wiki/src/about.es.po
	wiki/src/doc.de.po
	wiki/src/doc.fr.po
	wiki/src/doc/about/warning.de.po
	wiki/src/doc/about/warning.es.po
	wiki/src/doc/about/warning.fr.po
	wiki/src/doc/use/encryption/truecrypt.mdwn
	wiki/src/support/walkthrough.mdwn
	wiki/src/todo/restructure_the_website_navigation.mdwn