doc: add .gitignore (#349)

When generating documentation, I import comps/version.py that causes a
__pycache__ directory to be created in my locally cloned GenAIComps
repo.  This pollutes my local repo as reported by git status.  By adding
a .gitignore for __pycache__ directories, they're properly ignored by
git.

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>

.gitignore

@@ -0,0 +1 @@
+__pycache__