docs: updating OEP-65 filename and adding link to merge discussion

[davidjoy]

• Adds "arch-" to OEP-65's filename
• Adds a link to the slack discussion in #open-edx-proposals where we decided to merge the OEP as provisional.

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[sarina]

Ah yeah @davidjoy I remember why I wanted you to just close the PR and open a new one. The failing check is because you moved a file and now have to set up a redirect.

[davidjoy]

Ugh! Done. Hopefully that fixes it.

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