docs: Ignore FastTree link for linkcheck

[victorlin]

Description of proposed changes

This URL will occasionally fail and return 403 (broken).

Related issue(s)

• docs-ci: make linkcheck prone to transient network failures .github#106

Checklist

• Automated checks pass
• Check if you need to add a changelog message
• Check if you need to add tests
• Check if you need to update docs

[victorlin]

Link is ignored:

(installation/installation: line    9) -ignored- http://www.microbesonline.org/fasttree/

[corneliusroemer]

Yes please :)

[corneliusroemer]

I'll merge this already now as it keeps coming up and is a pain, hope this is ok @victorlin

[victorlin]

Yes, thanks for merging!