manual: increase retry count for linkcheck

`linkcheck_retries` defaults to 1 [1], which isn't great for running
checks across the Internet, on a few dozen different servers, across
thousands of CI runs.

This wasn't tested, but my naïve expectation is that the option works as
documented. Not knowing what retry timeout is used is a bit unsettling,
but I guess we'll just see if this helps.

[1] - https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-linkcheck_retries

docs/manual/src/conf.py

@@ -87,3 +87,9 @@
     # React page with README content included as a JSON payload.
     r"^https://github\.com/[^/]+/[^/]+/$",
 ]
+
+# Defaults to one, which is a bit harsh and results in flakiness. This can be
+# probably bumped up even further if needed, but at some point this might not
+# enough and linkcheck would likely require some changes to work better in a CI
+# scenario (better controlled backoff, a degree of liveness caching, etc).
+linkcheck_retries = 3