docs: ignore pycache directory created by custom spelling filters

We recently added a custom filter for spell checking, in order to have
better control on the case for some specific terms. This custom filter
is written as a Python function in a dedicated file, and it turns out
that this file is pre-compiled when building the documentation. Let's
add the relevant __pycache__ directory to the .gitignore in the
documentation directory.

Fixes: 8710477 ("docs: add custom spelling filter to check WireGuard spelling")
Fixes: #16513

Signed-off-by: Quentin Monnet <quentin@isovalent.com>

Documentation/.gitignore

@@ -1,5 +1,6 @@
 _build
 _api
 _preview
+_exts/__pycache__
 Pipfile
 Pipfile.lock