docs: add custom spelling filter to check WireGuard spelling

In the documentation, WireGuard should always be spelt with capital W
and G. We also have a lowercase version coming from the documentation
for Helm value, where "wireguard" is a value that one can pass to a
dedicated option to select the encryption mode.

How to enforce correct spell checks for WireGuard, to avoid to introduce
occurrences with erroneous capitalisation?

- If the spelling list contains "wireguard" or "Wireguard", the check
  will be case-insensitive and "Wireguard" will not trigger an error
  (false negatives).

- If the spelling list contains "WireGuard" only, then the Helm value
  will trigger an error (false positive).

We want to allow both "WireGuard" and "wireguard". This cannot be done
via the spelling list, but can be implemented as a custom filter for
Sphinx's spelling extension, in order to skip either of those spellings.
Let's create this filter to make sure we capitalise the term correctly
in the future.

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

Documentation/_exts/cilium_spellfilters.py

@@ -0,0 +1,9 @@
+from enchant.tokenize import Filter
+
+class WireGuardFilter(Filter):
+    """Accept either 'wireguard' (for documenting Helm values) or 'WireGuard',
+    but not 'Wireguard'.
+    """
+
+    def _skip(self, word):
+        return (word == 'wireguard' or word == 'WireGuard')

Documentation/conf.py

@@ -17,11 +17,13 @@
 # documentation root, use os.path.abspath to make it absolute, like shown here.
 #
 import os
-# import sys
+import sys
 import re
 import subprocess
 import semver
-# sys.path.insert(0, os.path.abspath('.'))
+
+sys.path.insert(0, os.path.abspath('_exts'))
+import cilium_spellfilters
 
 
 # -- General configuration ------------------------------------------------
@@ -151,6 +153,9 @@
 # If true, `todo` and `todoList` produce output, else they produce nothing.
 todo_include_todos = False
 
+# Add custom filters for spell checks.
+spelling_filters = [cilium_spellfilters.WireGuardFilter]
+
 
 # -- Options for HTML output ----------------------------------------------
 

Documentation/spelling_wordlist.txt

@@ -898,7 +898,6 @@ whitelists
 whitespace
 wildcard
 wildcards
-wireguard
 workflow
 workflows
 workspace