[EPIC] Consolidate all maintainer process docs in new "Maintainer Docs" page

### Is this a duplicate?

- [x] I confirmed there appear to be no [duplicate issues](https://github.com/NVIDIA/cccl/issues) for this bug and that I agree to the [Code of Conduct](CODE_OF_CONDUCT.md)

### Is this for new documentation, or an update to existing docs?

New

### Describe the incorrect/future/missing documentation

## Summary
CCCL maintainer process/policy documentation is currently fragmented across:
- rendered docs (`docs/*.rst`)
- top-level markdown docs (`CONTRIBUTING.md`, `ci-overview.md`, `AGENTS.md`, etc.)
- workflow/config files (`.github/workflows/*.yml`, issue/PR templates)
- GitHub wiki process pages

This issue tracks consolidation into a single canonical Maintainer Docs section under `docs/maintainers/`, with clear contributor-vs-maintainer boundaries and de-duplication of stale content.

## Why
- Reduce duplicated/outdated process guidance
- Make maintainer workflows discoverable and reviewable
- Keep executable workflow truth in YAML/scripts, but make human policy truth live in docs

## References
- RAPIDS Maintainer Docs: https://docs.rapids.ai/maintainers/
- RAPIDS Contributing docs: https://docs.rapids.ai/contributing/
- CCCL Wiki home/process docs: https://github.com/NVIDIA/cccl/wiki

## Goals
- Establish `docs/maintainers/*` as the canonical maintainer process docs.
- Migrate relevant content from docs/wiki/markdown/workflow comments.
- Add links from old locations to canonical pages.
- Deprecate or archive stale duplicate pages.

## Proposed Maintainer Docs
- [x] Branching Strategy
- [x] Backport Process
- [ ] Release Management
- [ ] Triage and Issue Lifecycle
- [ ] PR and Code Review
- [ ] Project Board Usage
- [ ] CI
- [ ] Build/Test
- [ ] Policy and Governance References

## Source Inventory
### Existing docs pages
- `docs/cccl/development/index.rst`
- `docs/cccl/development/build_and_bisect_tools.rst`
- `docs/cccl/contributing.rst`
- `docs/thrust/release_process.rst`
- `docs/thrust/releases.rst` / `docs/cub/releases.rst` / `docs/libcudacxx/releases.rst`

### Top-level/process markdown
- `CONTRIBUTING.md`
- `ci-overview.md`
- `.devcontainer/README.md`
- `CODE_OF_CONDUCT.md`, `SECURITY.md`

### Workflow/config-embedded policy
- Release workflows + docs:
- `.github/workflows/release-README.md`
- `.github/workflows/release-create-new.yml`
- `.github/workflows/release-update-rc.yml`
- `.github/workflows/release-finalize.yml`
- `.github/workflows/release-wheels.yml`
- `.github/workflows/update-branch-version.yml`
- `.github/release.yml`
- Backporting:
- `.github/workflows/backport-prs.yml`
- Triage/project automation:
- `.github/workflows/triage_rotation.yml`
- `.github/workflows/project_automation_*.yml`
- Contribution interfaces:
- `.github/PULL_REQUEST_TEMPLATE.md`
- `.github/ISSUE_TEMPLATE/*.yml`
- `.github/copy-pr-bot.yaml`

### Wiki pages to migrate
- Home / Principles for Open Development
- Triage
- Issue and PR Hygiene
- Issue Types
- Project Usage
- Sprint Planning
- Roadmapping/Release Planning
- Code Review

### If this is a correction, please provide a link to the incorrect documentation. If this is a new documentation request, please link to where you have looked.

_No response_