Update Documentation URLs

[mrmundt]

Fixes NA

Summary/Motivation:

There were a few missed URLs after the doc reorg that were pointing to stable (which was officially updated as of last Friday when 6.8.1 came out).

Changes proposed in this PR:

• Fix residual broken URLs

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[jsiirola]

Please update the RST files to use :ref: directives and not full URLs (that will future-proof them)

Apart from that, this is fine, but I wonder if a better (long term) solution is for the Sphinx build to declare a |build| substitution variable (with rst_prolog) that resolves either to latest or stable so that the links automatically point to the correct location?

We might have to update the URL checker to recognize /expand those URLs though?

requested changes have been made