Docs: Fixed broken link in location guidance

[mrshll1001]

This PR introduces a non-normative change to the docs and therefore doesn't have any versioning implications.

Changed link on the Location Guidance page from:

• https://standard.threesixtygiving.org/en/latest/guidance/location-guide/#why-is-beneficiary-location-data-useful

to

• https://help.grantnav.threesixtygiving.org/en/latest/#location-pages

As per Planio #50521

The branch is building properly thanks to changes introduced in main:

• Build on readthedocs
• Changed page on docs branch