Document mail template placeholders in White labeling

[hophead12]

Description

The White labeling > Mail templates page listed the available templates but said
nothing about the placeholders you can use inside them, so there was no way to
tell from the docs which variables a given template accepts.

This adds:

• an Available placeholders column to the template table, listing the
  placeholders each template provides;
• a Placeholders section describing what every placeholder resolves to;
• the Test email message template, which was missing from the table.

The template rows are also renamed to match the labels shown in the
Mail template drop-down verbatim (Account activation message instead of
Activation, and so on), so the table can be read side by side with the UI.

Two behaviours are called out explicitly because they are easy to get wrong and
both break silently:

• Placeholders are substituted only in the mail body. The subject is sent as
  plain text, so ${...} in a subject reaches the recipient literally.
• A placeholder the template does not provide breaks that template entirely.
  Rendering fails, so no email of that type is sent at all — for any user, and
  permanently, until the placeholder is removed. There is no fallback to the
  default template, and the placeholder is not left in the text as-is. A typo in
  a placeholder name has exactly the same effect.

Everything was verified against the platform sources: the FreeMarker model built
per template in DefaultMailService, the shipped defaults in
mail_templates.json (whose bodies use exactly those variables and no others),
and the UI labels in locale.constant-en_US.json.

The three Api usage state templates are intentionally left out: the
GET /api/whiteLabel/mailTemplates endpoint strips them from its response for
every authority, so they never appear in the drop-down and cannot be configured
here.

Type of change

• Shared include file (src/content/_includes/**)

Affected products

PE, PaaS and PaaS/EU — the page is a PE feature, and all four product stubs
(CE, PE, PaaS, PaaS/EU) share the single include that changed, so the new
content appears on all of them.

Related issues

None.

Checklist

• pnpm check passes (Astro / TypeScript) — 0 errors, 0 warnings, 0 hints
• pnpm lint:eslint passes
• pnpm lint:slugcheck passes
• pnpm lint:linkcheck — not run. No pages were added, renamed or removed
  and no links were touched (the one DocLink on the page is unchanged), so
  there is nothing new for it to resolve. Happy to run it if you'd like.
• Renamed/removed pages have a redirect — n/a, no pages renamed or removed
• No hardcoded versions
• Screenshots attached for visual changes — n/a, no new images; the two
  existing screenshots are unchanged

[Copilot]

Pull request overview

Updates the ThingsBoard documentation for White labeling → Mail templates to clearly document which FreeMarker placeholders are supported by each template, helping admins avoid silent template rendering failures when customizing email bodies.

Changes:

• Expanded the mail template table with an Available placeholders column and aligned template names with the UI labels.
• Added a new Placeholders section explaining substitution behavior (body-only) and documenting each placeholder’s meaning.
• Added the missing Test email message template entry.

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