Bug 1421241 - Docs: Improve livehtml mode and recommend it

* Removes the unnecessary initial `make html`, since sphinx-autobuild
performs the initial build itself.
* Enables `--poll` watch mode, since file change detection otherwise
doesn't work in Vagrant.
* Updates the docs to recommend the livereload web server over the
one-off build when working on docs locally.

docs/Makefile

@@ -24,5 +24,4 @@ html:
 	@echo "Build finished. The HTML pages are in $(BUILDDIR)."
 
 livehtml:
-	make html
-	sphinx-autobuild -b html $(SPHINXOPTS) $(BUILDDIR)
+	sphinx-autobuild -b html --poll $(SPHINXOPTS) $(BUILDDIR)

docs/common_tasks.rst

@@ -95,15 +95,15 @@ Building the docs locally
 -------------------------
 
 * Either ``vagrant ssh`` into the VM, or else activate a virtualenv on the host machine.
-
 * From the root of the Treeherder repo, run:
 
   .. code-block:: bash
 
      > pip install -r requirements/docs.txt
-     > make -C docs html
+     > make -C docs livehtml
 
-* The built docs can then be found inside ``docs/_build/``.
+* Visit http://127.0.0.1:8000 to view the docs.
+* Source changes will result in automatic rebuilds and browser page reload.
 
 
 Sharing UI-only changes with others using GitHub Pages