doc/userguide: add appendix item on eve schema

Add some basic documentation on our EVE schema, mainly to show users
that it exists.

doc/userguide/appendix/eve-schema.rst

@@ -0,0 +1,80 @@
+EVE JSON Schema
+###############
+
+The Suricata source distribution contains a JSON schema for the EVE
+log files. This schema follows the `JSON Schema
+<https://json-schema.org/>`_ specification and can be found in
+``etc/schema.json``. If your distribution does not contain this file,
+it can be viewed online at
+https://github.com/OISF/suricata/blob/master/etc/schema.json, but note
+that it is version-specific and may change between major versions of
+Suricata.
+
+This schema attempts to log all possible fields that may be seen in
+Suricata's **EVE** output, including their datatype. It also includes
+extensions to help map log fields to related detection keywords.
+
+Suricata Schema Extensions
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+We have extended JSON schema with a ``suricata`` object to add extra
+Suricata context such as detection keywords related to a log field,
+for example:
+
+.. code-block:: json
+
+   "rrname": {
+       "type": "string",
+       "suricata": {
+           "keywords": [
+               "dns.answers.rrname",
+               "dns.response.rrname"
+           ]
+       }
+   }
+
+The above shows that a field named ``rrname`` has 2 keywords that are
+related. Please refer to the keyword documentation to see precisely
+how they are used and related to the field being logged.
+
+Extension Reference
+===================
+
+The ``suricata`` extension object is valid on objects inside the
+``properties`` object. The ``suricata`` object may accept the
+following fields:
+
+``keywords``
+------------
+
+**Type:** ``array`` or ``boolean``
+
+* **When an array:** Contains keyword names that are related to this
+  JSON property. Each keyword in the array represents a detection rule
+  keyword that can be used to match against the corresponding field
+  value.
+
+* **When ``false``:** Indicates that this JSON property has no
+  applicable keyword. This is used for metadata fields that don't
+  correspond to actual network data. For example, the ``version``
+  field inside a DNS object denotes the version of the log format and
+  is unrelated to any aspect of a DNS message, therefore no keyword is
+  applicable.
+
+.. note:: As of Suricata 8.0, mapping log fields to detection keywords
+          is a work in progress. Any field that does not have a
+          ``suricata.keywords`` value still needs to be evaluated.
+
+Schema Tooling
+^^^^^^^^^^^^^^
+
+* `Suricata-Verify <https://github.com/OISF/suricata-verify>`_: Our
+  own tool for verifying every Suricata pull request, validates all
+  EVE logs generated against the schema.
+
+* ``./scripts/eve-parity.py``: Found inside the Suricata source code
+  when checked out with ``git``, is a tool to provide information on
+  how log fields map to keywords, or how keywords map to log entries.
+
+* ``./scripts/evedoc.py``: Generate documentation from the schema,
+  such as the :doc:`eve-index` included in this documentation.

doc/userguide/appendix/index.rst

@@ -4,4 +4,5 @@ Appendix
 .. toctree::
    :maxdepth: 1
 
+   eve-schema
    eve-index