docs: tag Kibana custom branding availability on serverless (#8195)

[florent-leborgne]

Summary

Identified by @nastasha-solomon's eagle eyes - part of the settings became available in serverless earlier this summer and only the advanced settings page got updated.

This PR addresses #8195 with the following changes:

• deploy-manage/kibana-custom-branding.md: Marks the page as available on serverless, tags Page title as serverless-unavailable, and drops deployment-only wording from the intro so the steps match Elastic Stack and Elastic Cloud Serverless.

The companion Advanced Settings reference in elastic/kibana (docs/reference/advanced-settings-global.yml) already tags the same split: logo, organization name, and both favicons are serverless GA, and xpackCustomBranding:pageTitle is serverless unavailable. No Kibana companion PR is needed.

Source: serverless allowlist in src/platform/packages/shared/serverless/settings/common/index.ts (added in elastic/kibana#279246). Page title is not on that list or any project-specific allowlist.

Resolves

Closes #8195

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🔍 Preview links for changed docs

• deploy-manage/kibana-custom-branding.md

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✅ Elastic Docs Style Checker (Vale)

No issues found on modified lines!

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The Vale linter checks documentation changes against the Elastic Docs style guide. To use Vale locally or report issues, refer to Elastic style guide for Vale.

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Docs review summary

Focus areas

• Style and clarity: Vale reported no issues. One phrasing nit below.
• Jargon: None found.
• Frontmatter and applies_to: applies_to change (serverless: ga at page level, inline {applies_to}serverless: unavailable`` on Page title) is correctly scoped per the cumulative-docs syntax reference and matches the companion elastic/kibana `docs/reference/advanced-settings-global.yml`, which tags logo/organization name/both favicons as `serverless: ga` and `xpackCustomBranding:pageTitle` as `serverless: unavailable`.
• Content type fit: No issues; how-to structure unchanged.
• Contradictions: No contradictions found. Cross-checked the published Kibana advanced settings reference and the elastic/kibana YAML source; both confirm the exact availability split this PR describes.
• Parent issue satisfaction: Not applicable (no linked issue; PR description states the motivating context directly).

Nits

• Line 18: "Replace the default Elastic logo, organization name, and more with your own custom assets." — "and more" is vague where the previous wording named the specific assets (logo, organization name, browser tab title, favicon). Since page title is now serverless-unavailable rather than universally available, consider naming the still-universal assets explicitly (logo, organization name, favicon) instead of the generic "and more," so readers know what's actually configurable without opening the page-title bullet below.

Notes

• Verified the technical claim (serverless allowlist split between logo/org-name/favicons vs. page title) directly against elastic/kibana's docs/reference/advanced-settings-global.yml, which matches the PR description.

Generated by Docs review agent for #8190 · copilot · sonnet50 · 54.3 AIC · ⌖ 18.9 AIC · ⊞ 18.1K · ◷

[nastasha-solomon]

** Agent Smith voice ** Very good, Mr. Le Borgne.