Increase docs-linkcheck worker threads, ignore SD issues URLs
Increase docs-linkcheck worker threads, ignore SD issues URLs

We hit URLs under https://github.com/freedomofpress/securedrop/issues
more than GitHub likes, resulting in 429 Too Many Requests
errors. These URLs aren't likely to change or go missing, so ignoring
them seems a safe way to prevent spurious linkcheck failures.

Also increase the number of worker threads checking URLs, to reduce
the overall time linkcheck takes.

conorsch/securedrop,conorsch/securedrop,conorsch/securedrop,conorsch/securedrop,conorsch/securedrop