[Email Service] Add Activity log message preview changelog and docs

[ttoino]

Summary

EMAIL-1964 — documents the new Email Service message preview feature in the sending Activity log.

• New changelog entry (dated 2026-07-17) announcing that sent emails can be
  previewed — HTML, Text, Headers, Attachments, and Raw source — from the Activity log.
• Adds a Message preview section to the Email logs page (observability/logs.mdx).
• Adds an Email preview section to the sending domains page (configuration/domains.mdx),
  covering the per-domain setting, ~7-day retention, and auto-enable for domains
  onboarded on/after 2026-07-02.

Documentation checklist

• Is there a changelog entry (guidelines)?
• The change adheres to the documentation style guide.

[cloudflare-docs-bot]

Review

💡 1 suggestion found in commit cf20d79.

Fix in your agent

Fix the following review findings in PR #32129 (https://github.com/cloudflare/cloudflare-docs/pull/32129).

Before making changes, review each finding and present a brief summary table:
- For each finding, state whether you agree, disagree, or need clarification
- If you disagree (e.g. the fix requires disproportionate effort for minimal benefit,
  or the finding is factually incorrect), explain why
- If you need clarification before deciding, ask those questions
- Then share your plan for which issues to tackle and in what order

After triaging, fix all legitimate findings. For any you decide to skip,
post a comment on this PR with the finding ID and your reasoning.

---

## Style Guide Review

### Suggestions (1)

#### SG-bb2c22120aa4 · Spell out single-digit numerals in body text
- **File:** `src/content/changelog/email-service/2026-07-17-email-message-preview.mdx` line 15
- **Issue:** Line adds `about 7 days`
- **Fix:** Change to `about seven days`

Code Review

This code review is in beta and may not always be helpful — use your judgment.

_{No code review issues found.}

Conventions

_{No convention issues found.}

Style Guide Review

Suggestions (1)

File	Issue
changelog/email-service/2026-07-17-email-message-preview.mdx line 15	Spell out single-digit numerals in body text — Line adds about 7 days Fix: Change to about seven days

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