doc: Refactors the documentation

This commit refactors the documentation according to my proposal in #1273
and the discussion I had with fd0 on IRC.

The bits from the manual that I could not immediately put into the new
structure are contained in manual_rest.rst Anything else is still there,
nothing has been deleted.

I changed the heading markup to follow the convention used in Python’s
Style Guide for documentation, this convention is explained in a comment
at the top of every file.

I also added a paragraph on installing restic on Debian.

doc/070_encryption.rst

@@ -0,0 +1,51 @@
+..
+  Normally, there are no heading levels assigned to certain characters as the structure is
+  determined from the succession of headings. However, this convention is used in Python’s
+  Style Guide for documenting which you may follow:
+
+  # with overline, for parts
+  * for chapters
+  = for sections
+  - for subsections
+  ^ for subsubsections
+  " for paragraphs
+
+##########
+Encryption
+##########
+
+
+*"The design might not be perfect, but it’s good. Encryption is a first-class feature,
+the implementation looks sane and I guess the deduplication trade-off is worth it. So… I’m going to use restic for 
+my personal backups.*" `Filippo Valsorda`_
+
+.. _Filippo Valsorda: https://blog.filippo.io/restic-cryptography/
+
+**********************
+Manage repository keys
+**********************
+
+The ``key`` command allows you to set multiple access keys or passwords
+per repository. In fact, you can use the ``list``, ``add``, ``remove``
+and ``passwd`` sub-commands to manage these keys very precisely:
+
+.. code-block:: console
+
+    $ restic -r /tmp/backup key list
+    enter password for repository:
+     ID          User        Host        Created
+    ----------------------------------------------------------------------
+    *eb78040b    username    kasimir   2015-08-12 13:29:57
+
+    $ restic -r /tmp/backup key add
+    enter password for repository:
+    enter password for new key:
+    enter password again:
+    saved new key as <Key of username@kasimir, created on 2015-08-12 13:35:05.316831933 +0200 CEST>
+
+    $ restic -r backup key list
+    enter password for repository:
+     ID          User        Host        Created
+    ----------------------------------------------------------------------
+     5c657874    username    kasimir   2015-08-12 13:35:05
+    *eb78040b    username    kasimir   2015-08-12 13:29:57