[docs-patrol] Update Heartbeat browser docs for certificate_error_spki_allowlist

Recent code changes introduced a new Heartbeat browser monitor configuration option, but the current docs do not describe it.

## Changes Requiring Documentation Updates

### 1. New `certificate_error_spki_allowlist` browser option is undocumented

**Commit(s):** [`e456e143e6bba6a826324a0639f8f31c7a7a1b0c`](https://github.com/elastic/beats/commit/e456e143e6bba6a826324a0639f8f31c7a7a1b0c)

**What changed:**
- `x-pack/heartbeat/monitors/browser/config.go:47` adds `config:"certificate_error_spki_allowlist"`.
- `x-pack/heartbeat/monitors/browser/sourcejob.go:168-170` forwards the value to synthetics CLI as `--certificate-error-spki-allowlist`.

**Documentation impact:**
- `docs/reference/heartbeat/configuration-heartbeat-options.md:89-93` points users to generic browser-monitor guidance and a legacy 8.7 page, but there is no mention of `certificate_error_spki_allowlist` (or `certificate-error-spki-allowlist`) in current repo docs.
- This leaves a new user-facing monitor option undiscoverable from current Heartbeat documentation.

## Suggested Actions

- [ ] Add `certificate_error_spki_allowlist` to the current Heartbeat browser monitor docs/reference location, including expected values and behavior.
- [ ] Document that the corresponding Chromium flag is omitted for API journeys (as implemented in `x-pack/heartbeat/monitors/browser/sourcejob.go:166-171`).

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Start with x-pack/heartbeat/monitors/browser/config.go:47 and sourcejob.go:166-171, then inspect docs/reference/heartbeat/configuration-heartbeat-options.md:89-93. Update the current Heartbeat browser-monitor documentation with the option’s expected values and behavior, including that the Chromium flag is omitted for API journeys. Done means users can discover and understand certificate_error_spki_allowlist in the current docs.

Written by the indexing model from the issue text.