fetch-service: added inspectors and certificates docs

Added proper documentation for inspectors configuration and the certificate installation.

reference/services/fetch-service.rst

@@ -100,10 +100,64 @@ We deploy the fetch service using the specs defined in
 `fetch service mojo specs <https://git.launchpad.net/~launchpad/launchpad-mojo-specs/+git/private/+ref/master>`_.
 
 In order to be able to evaluate new fetch service versions, we use different
-Snap channels for qastaging and production, so we are able to
+Snap channels and revisions for qastaging and production, so we are able to
 test new releases. This information is both defined in above mentioned mojo
 specs, and in `ST118 fetch service release process <https://docs.google.com/document/d/1HZvFo78LqFGgdpM7v3teG9gV-pMyvXpXTD1vcLLv_d0/>`_.
 
+The fetch service uses various inspectors: we have the inspector for ``git``, ``craft`` builds
+and other will be released in future. 
+The inspector is responsible for inspecting the requests and the various
+downloads that are made during the build. The inspector is also responsible
+for making sure that the requests are allowed to be made.
+Every inspector is fully configurable and the ``allowlist`` is specified in the configuration
+file.
+This configuration is stored in the aforementioned specs.
+
+Configuration example:
+
+.. code-block:: yaml
+
+   git:
+     urls:
+       - https://test.com/**
+
+   crafts:
+     urls:
+      - https://test.com/**
+
+   snap:
+     snap-declaration:
+       - name: publisher-id
+         value: [canonical]
+
+   apt:
+     repositories:
+       default:
+         urls:
+           - http://archive.ubuntu.com/ubuntu
+         dists:
+           - "*"
+         components:
+           - "*"
+
+
+Moreover, the fetch service snap require certificates to work properly.
+This is something that the snap can create when it's installed. 
+
+These certificates are also configurable from the charm itself if we have the need to
+change them, using the following command:
+
+``juju config fetch-service proxy.certificate="$(cat certs/ca.pem)" proxy.key="$(cat certs/ca.key.pem)"``
+
+The certificates are stored in the ``${SNAP_DATA}/certs`` directory inside the fetch-service
+charm unit.
+
+In order to configure properly our builders and the ``launchpad-buildd-manager`` we should 
+pass the ``base64`` encoded ``ca.pem`` certificate to the ``launchpad-buildd-manager`` charm 
+using the following command, making sure that is passed as a one-line value:
+
+``juju config launchpad-buildd-manager fetch_service_mitm_certificate=<encoded-one-line-value>``
+
 Qastaging
 ~~~~~~~~~
 For qastaging deployment, SSH into