Release versions 8.7/3.15.0

[pepopowitz]

Release new versions of the documentation: 8.7 for the main docs, 3.15.0 for the Optimize docs.

Due date: TBD.

See #4158 for the previous release, for more details on each step.

The week before (part of this issue)

Preview Give feedback

1. Communicate in Slack the schedule for cutting docs, multiple times. Example.
2. synchronize sidebars for all actively supported versions
3. do a practice cut, to uncover unexpected issues in advance

The day before (part of this issue)

Preview Give feedback

1. Communicate to #ask-documentation that cutting is beginning, and changes merged after this point may need to be backported manually.
2. cut 8.7/3.15.0 version (in two parts: reviewable changes, and a massive PR with all the new content.)
3. Publish docs release 8.7.0
4. initiate Algolia crawl
5. initiate Kapa ingestion

Follow-up Tasks (part of this issue, should be done within a week of release)

Preview Give feedback

1. submit sitemap.xml

Follow-up Tasks (non-blocking to this issue, begin this work shortly after release)

Preview Give feedback

1. Archive versions 8.3/3.11.0 (if the release occurs when their support ends.)
2. re-synchronize sidebars

Acceptance criteria

• The version dropdown reads "8.7" for the main docs instance.
• The version dropdown reads "8.7 / 3.15.0" for the Optimize docs instance.
• An "unmaintained" banner displays for 8.3/3.11.0, if they have exceeded support.
• No "unmaintained" banner should display for versions 8.4/3.12, 8.5/3.13, 8.6/3.14, or 8.7/3.15.
• An "unreleased" banner should display for versions Next.

[pepopowitz]

cc @camunda/tech-writers, in case you want to attach yourself as an assignee to be more involved in this release.