docs: Mark OEP-48 as Approved and OEP-23 as replaced

[xitij2000]

OEP-48 deals with customising branding for MFEs usin an npm package override.
This mechanism for branding is now supported across all MFEs, and is also
supported by both the configuration repo, and Tutor. As such this PR updates its
status from Provisional to Accepted.

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[kdmccormick]

Thanks for making this change @xitij2000 , I agree it's time to mark this as complete. Could you:

• mark OEP-23 as "Replaced"
• add a changelog entry to the bottom of this OEP as well as OEP-23
• post this PR in the #open-edx-proposals channel and wait a week, just to give folks a chance to raise their hand if they disagree

[xitij2000]

Thanks for making this change @xitij2000 , I agree it's time to mark this as complete. Could you:

* mark OEP-23 as "Replaced"

* add a changelog entry to the bottom of this OEP as well as OEP-23

* post this PR in the #open-edx-proposals channel and wait a week, just to give folks a chance to raise their hand if they disagree

Sorry for the delay. I've updated the PR.

[kdmccormick]

Thanks again!

[itsjeyd]

Hey @kdmccormick and @arbrandes! Now that this PR is ready to go, would it be possible to move forward with merging it? I'm not sure who has permission to hit the button; please let me know if there's someone else I should ping.

CC @mphilbrick211

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