[mod] make docs-live - remove --port option from sphinx-autobuild

Setting port fixed to 8080 has drawbacks when this port is already in use from
another application [1].  Removing this option from the command line offers the
use of the sphinx-autobuild defaults which are much more flexible [2].

[1] https://github.com/searx/searx/issues/2282
[2] https://github.com/executablebooks/sphinx-autobuild/blob/master/README.md

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

docs/dev/contribution_guide.rst

@@ -141,6 +141,9 @@ Here is an example which makes a complete rebuild:
 live build
 ----------
 
+.. _sphinx-autobuild:
+   https://github.com/executablebooks/sphinx-autobuild/blob/master/README.md
+
 .. sidebar:: docs-clean
 
    It is recommended to assert a complete rebuild before deploying (use
@@ -156,9 +159,21 @@ changed.
    $ make docs-live
    ...
    The HTML pages are in dist/docs.
-   ... Serving on http://0.0.0.0:8080
+   ... Serving on http://0.0.0.0:8000
    ... Start watching changes
 
+Live builds are implemented by sphinx-autobuild_.  Use environment
+``$(SPHINXOPTS)`` to pass arguments to the sphinx-autobuild_ command.  Except
+option ``--host`` (which is always set to ``0.0.0.0``) you can pass any
+argument.  E.g to find and use a free port, use:
+
+.. code:: sh
+
+   $ SPHINXOPTS="--port 0" make docs-live
+   ...
+   ... Serving on http://0.0.0.0:50593
+   ...
+
 
 .. _deploy on github.io:
 

utils/makefile.sphinx

@@ -79,7 +79,7 @@ quiet_cmd_sphinx = SPHINX    $@ --> file://$(abspath $(DOCS_DIST)/$5)
 	-b $2 -c $3 -d $(DOCS_BUILD)/.doctrees $4 $(DOCS_DIST)/$5
 
 quiet_cmd_sphinx_autobuild = SPHINX    $@ --> file://$(abspath $(DOCS_DIST)/$5)
-      cmd_sphinx_autobuild = PATH="$(PY_ENV_BIN):$(PATH)" $(PY_ENV_BIN)/sphinx-autobuild  $(SPHINX_VERBOSE) --open-browser --host 0.0.0.0 --port 8080 $(SPHINXOPTS)\
+      cmd_sphinx_autobuild = PATH="$(PY_ENV_BIN):$(PATH)" $(PY_ENV_BIN)/sphinx-autobuild  $(SPHINX_VERBOSE) --open-browser --host 0.0.0.0 $(SPHINXOPTS)\
 	-b $2 -c $3 -d $(DOCS_BUILD)/.doctrees $4 $(DOCS_DIST)/$5
 
 quiet_cmd_sphinx_clean = CLEAN     $@