docs: release v0.15.0

[jm-clius]

Adds the changelog for Release v0.15.0

@LNSD I have tentatively added release notes for the Relay RPC changes, in case the corresponding PR is merged before the release. If not, we can simply remove these items from the release notes before cutting the release.

Note the final release notes will include Upgrade Instructions:

• for the Relay RPC API breaking change
• to explain the impact of changing how message IDs are computed (e.g. running older versions of nwaku on the same network will cause duplication)

For the items you were responsible for, please check:

• that it's included in the release notes
• that you're happy with the phrasing

[LNSD]

I just opened for code review the JSON-RPC API. I think it can make it as part of the release 😁

[rymnc]

LGTM ~ there's a fix in #1559 that should probably be a part of this release, if possible!

[alrevuelta]

lgtm!

would like this PR to be part of the release (if it wasnt considered) #1554. It fixes a prune peerstore issue.

[jm-clius]

would like this PR to be part of the release (if it wasnt considered)

Will be part of the release! Since Prune peers from peerstore when exceeding capacity is introduced in this release and in the notes, I didn't include a separate release note for this issue as it can be seen as a further increment/fix on the same feature.

[LNSD]

LGTM ✅ 🏁