docs: update repo purpose to status accepted

[mariajgrimaldi]

Description

This PR updates the repository purpose ADR as accepted since this solution has been used for a couple of years.

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[mariajgrimaldi]

There are other ADRs with out-of-date statuses, but they should be handled in other PRs once we figure out the state of the proposals. Here are the comments I've been leaving to do so:
#40 (comment)
#268 (comment)