docs: update capability documentation links

[SirRGB]

What does this PR aim to accomplish?:
Old links refer to this github readme, but the section about that has been removed in pi-hole/docker-pi-hole@ebe5799

How does this PR accomplish the above?:
Refer to the pihole docs instead

Link documentation PRs if any are needed to support this PR:
pi-hole/docker-pi-hole#2025 (merged)

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[SirRGB]

It may also be reasonable to remove the "More info at https://github.com/pi-hole/docker-pi-hole/" part, because that readme has less information than the docs and the readme also refers to the docs for more info..