docs(temporal-proxy): add Temporal Proxy configuration page (#4954)

* docs(temporal-proxy): add Temporal Proxy configuration page

Add a deploy to production page for the Temporal Proxy, the gRPC proxy
that routes SDK Client and Worker traffic to one or more upstream
Temporal Services (local, self-hosted, or Temporal Cloud) while handling
Namespace translation, TLS, and credentials.

The proxy is pre-release, so the page leads with that caveat and links
to the repository, the Cloud example, and the Helm chart for the
definitive, still-evolving schema. Lives under production-deployment as
a directory (index.mdx) to leave room for future pages; the slug is
pinned so the URL is stable regardless of file location.

* docs(temporal-proxy): address review feedback on slug, release header, and Vale headings

Remove the redundant slug (matches the auto-generated path per CONTRIBUTING.md's
page-routing guidance), switch the manual pre-release caution block to the
standard ReleaseNoteHeader component, and align tags/keywords with sibling
production-deployment pages.

Also extend Temporal.Headings exceptions for KMS and the exact product-name
phrases Azure Key Vault, Azure Workload Identity, and GCP Workload Identity,
resolving the open Vale false-positive threads on PR #4954.

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Co-authored-by: Duncan Mackenzie <duncanma@duncanmackenzie.net>
Co-authored-by: Duncan Mackenzie <duncan.mackenzie@temporal.io>

Author: pseudomuto