Updated some links from "relref" and "a href" to "ref".

[y82]

Proposed changes

This PR changes some (not all) "a href" and "relref" links to "ref" in /content/nginx. Tested with local preview.

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1. Potentially sensitive changes include anything involving code, personally identify information (PII), live URLs or significant amounts of new or revised documentation. Please refer to our style guide for guidance about placeholder content. ↩