docs: keep the RGW placement contract in HACKING.md for now

[johnramsden]

Description

RGW placement is still being built, so a published reference page would describe an API that can still change underneath it. The standing rules go where developers look instead: a new "RGW placement (in development)" section at the end of HACKING.md, which is not part of the Sphinx build under docs/ and so carries no promise of stability to an external reader.

The section documents the rgw object's fields with an example policy, the rules that hold today and what a PUT returns for RGW, the three on-member files (radosgw.conf, radosgw.conf.pending, rgw-tls/<hash>/), the upgrade limitation for a policy stored while rgw was still a boolean, and where the code lives.

Stack: PR 12 of 13. Merge in order. Each PR is one logical change on top of the one before it.

1. feat(db): add a table for each member's last applied RGW frontend #881 feat(db): add a table for each member's last applied RGW frontend
2. fix(services): keep error classes through the service enable path #882 fix(services): keep error classes through the service enable path
3. feat(api): describe an RGW placement as an object with explicit TLS intent #883 feat(api): describe an RGW placement as an object with explicit TLS intent
4. feat(rgw): make the member-side RGW apply safe to repeat #884 feat(rgw): make the member-side RGW apply safe to repeat
5. feat(rgw): wait until the gateway really serves the requested frontend #885 feat(rgw): wait until the gateway really serves the requested frontend
6. feat(rgw): reuse the member's own pair when TLS is asked for without one #886 feat(rgw): reuse the member's own pair when TLS is asked for without one
7. fix(rgw): make RGW disable converge when repeated or interrupted #887 fix(rgw): make RGW disable converge when repeated or interrupted
8. feat(placement): accept the RGW object in the policy and keep its secrets out #888 feat(placement): accept the RGW object in the policy and keep its secrets out
9. feat(placement): reconcile RGW across the cluster #889 feat(placement): reconcile RGW across the cluster
10. test(robot): add the single-VM RGW placement suite #890 test(robot): add the single-VM RGW placement suite
11. test(robot): add the three-VM RGW placement suite #891 test(robot): add the three-VM RGW placement suite
12. docs: keep the RGW placement contract in HACKING.md for now #892 docs: keep the RGW placement contract in HACKING.md for now (this PR)
13. fix(rgw): do not trust a pending-apply marker the operator completed by hand #893 fix(rgw): do not trust a pending-apply marker the operator completed by hand

Type of change

• Documentation update (change to documentation only)

How has this been tested?

Documentation only. Every statement was checked against the code in this stack.

Contributor checklist

Please check that you have:

• self-reviewed the code in this PR
• added code comments, particularly in less straightforward areas (not applicable, documentation only)
• checked and added or updated relevant documentation (this PR is the documentation)
• checked and added or updated relevant release notes
• added tests to verify effectiveness of this change (not applicable, documentation only)