linkchecker: Add ignores for housekeeping links

The Antora server will include a language-switching link
and an edit link for each file, both of which aren't
valid in the preview and need to be ignored.

The edit link takes the form `file:///antora/...`,
a path that's only valid inside the container.

The language link points from every document
`https://localhost:8080/guidelines/foo`
to a language-specific URL like
`https://localhost:8080/en-US/guidelines/foo`

The latter may need to be tweaked if they're dependent on
the system locale of the user running linkchecker.
Ignoring `https://localhost:8080/en-US` works for me,
but may not be universal.

Makefile

@@ -19,7 +19,9 @@ linkcheck:
 	$(docsbuilder) -k
 	$(docsbuilder) -p
 	linkchecker http://localhost:8080 --check-extern --no-robots \
-		--ignore-url "drupal.org"
+		--ignore-url "drupal.org" \
+		--ignore-url "file:///antora" \
+		--ignore-url 'http://localhost:8080/en-US'
 	$(docsbuilder) -k
 
 .DEFAULT_GOAL := doc