Add recoll engine (#2325)

recoll is a local search engine based on Xapian:
http://www.lesbonscomptes.com/recoll/

By itself recoll does not offer web or API access,
this can be achieved using recoll-webui:
https://framagit.org/medoc92/recollwebui.git

This engine uses a custom 'files' result template

set `base_url` to the location where recoll-webui can be reached
set `dl_prefix` to a location where the file hierarchy as indexed by recoll can be reached
set `search_dir` to the part of the indexed file hierarchy to be searched, use an empty string to search the entire search domain

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+.. _engine recoll:
+
+======
+Recoll
+======
+
+.. sidebar:: info
+
+   - `Recoll <https://www.lesbonscomptes.com/recoll/>`_
+   - `recoll-webui <https://framagit.org/medoc92/recollwebui.git>`_
+
+Recoll_ is a desktop full-text search tool based on Xapian. By itself Recoll_
+does not offer web or API access, this can be achieved using recoll-webui_
+
+
+
+Configuration
+=============
+
+You must configure the following settings:
+
+``base_url``:
+  Location where recoll-webui can be reached.
+
+``mount_prefix``:
+  Location where the file hierarchy is mounted on your *local* filesystem.
+
+``dl_prefix``:
+  Location where the file hierarchy as indexed by recoll can be reached.
+
+``search_dir``:
+  Part of the indexed file hierarchy to be search, if empty the full domain is
+  searched.
+
+
+Example
+=======
+
+Scenario:
+
+#. Recoll indexes a local filesystem mounted in ``/export/documents/reference``,
+#. the Recoll search inteface can be reached at https://recoll.example.org/ and
+#. the contents of this filesystem can be reached though https://download.example.org/reference
+
+.. code:: yaml
+
+   base_url: https://recoll.example.org/
+   mount_prefix: /export/documents
+   dl_prefix: https://download.example.org
+   search_dir: ''