docs: set gettext_uuid to False to reduce churn (#11567)

- `gettext_uuid=True` makes every commit update every .pot file in spack/localized-docs,
  and speeds up the internationalized doc build slightly.

- Optimize for less repository churn, and use `python-levenshtein` to accelerate 
  the build instead.

lib/spack/docs/conf.py

@@ -150,7 +150,7 @@ def setup(sphinx):
 
 # Sphinx gettext settings
 gettext_compact = True
-gettext_uuid = True
+gettext_uuid = False
 
 # There are two options for replacing |today|: either, you set today to some
 # non-false value, then it is used:

lib/spack/docs/requirements.txt

@@ -4,3 +4,4 @@
 sphinx==2.0.1
 sphinxcontrib-programoutput==0.14
 sphinx-rtd-theme==0.4.3
+python-levenshtein