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/045_working_with_repos.rst

@@ -0,0 +1,89 @@
+..
+  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
+
+
+#########################
+Working with repositories
+#########################
+
+Listing all snapshots
+=====================
+
+Now, you can list all the snapshots stored in the repository:
+
+.. code-block:: console
+
+    $ restic -r /tmp/backup snapshots
+    enter password for repository:
+    ID        Date                 Host    Tags   Directory
+    ----------------------------------------------------------------------
+    40dc1520  2015-05-08 21:38:30  kasimir        /home/user/work
+    79766175  2015-05-08 21:40:19  kasimir        /home/user/work
+    bdbd3439  2015-05-08 21:45:17  luigi          /home/art
+    590c8fc8  2015-05-08 21:47:38  kazik          /srv
+    9f0bc19e  2015-05-08 21:46:11  luigi          /srv
+
+You can filter the listing by directory path:
+
+.. code-block:: console
+
+    $ restic -r /tmp/backup snapshots --path="/srv"
+    enter password for repository:
+    ID        Date                 Host    Tags   Directory
+    ----------------------------------------------------------------------
+    590c8fc8  2015-05-08 21:47:38  kazik          /srv
+    9f0bc19e  2015-05-08 21:46:11  luigi          /srv
+
+Or filter by host:
+
+.. code-block:: console
+
+    $ restic -r /tmp/backup snapshots --host luigi
+    enter password for repository:
+    ID        Date                 Host    Tags   Directory
+    ----------------------------------------------------------------------
+    bdbd3439  2015-05-08 21:45:17  luigi          /home/art
+    9f0bc19e  2015-05-08 21:46:11  luigi          /srv
+
+Combining filters is also possible.
+
+
+Checking a repo's integrity and consistency
+===========================================
+
+Imagine your repository is saved on a server that has a faulty hard
+drive, or even worse, attackers get privileged access and modify your
+backup with the intention to make you restore malicious data:
+
+.. code-block:: console
+
+    $ sudo echo "boom" >> backup/index/d795ffa99a8ab8f8e42cec1f814df4e48b8f49129360fb57613df93739faee97
+
+In order to detect these things, it is a good idea to regularly use the
+``check`` command to test whether everything is alright, your precious
+backup data is consistent and the integrity is unharmed:
+
+.. code-block:: console
+
+    $ restic -r /tmp/backup check
+    Load indexes
+    ciphertext verification failed
+
+Trying to restore a snapshot which has been modified as shown above will
+yield the same error:
+
+.. code-block:: console
+
+    $ restic -r /tmp/backup restore 79766175 --target /tmp/restore-work
+    Load indexes
+    ciphertext verification failed
+