Linkcheck runtime improvement

During my experiments with AI, I was able to wring out a genuine improvement to the linkchecker runtime, with a simple configuration change. In my docs (which are large, and have well over a thousand links to check), the linkchecker takes about 10 minutes with the default settings currently in the SP:

```python
# give linkcheck multiple tries on failure
# linkcheck_timeout = 30
linkcheck_retries = 3
```

If the settings are changed instead to:

```python
# Give linkcheck multiple tries on failure
linkcheck_timeout = 15
linkcheck_retries = 2

# Number of parallel workers for linkcheck (default is 5)
# Higher values work well for network I/O-bound tasks
linkcheck_workers = 20
```

I cut down the runtime to 1.5 minutes (over 80% improvement).

I'm leaving this here as an issue rather than a PR because I'm not sure how the workers setting would impact other projects - the parallelism works well in the Server docs, but I could see a case for not including it in the SP if a project contains a high volume of links to a specific site, which could lead to linkcheck failures due to rate limiting. This part needs more general testing (but I recommend the timeout and retries both be reduced for the general performance improvement).