#3764 feat(integrations): add Mailtrap docs

Adds the missing integration page for Mailtrap. examples/mailtrap was added in
#3709 but never got a docs page, so Mailtrap doesn't show up in the Integrations
nav or in the overview cards.

Changes

• apps/docs/integrations/mailtrap.mdx: new integration page, registered in
  docs.json under Integrations.
• apps/docs/snippets/integrations.mdx: adds the Mailtrap card to the overview.

The page follows the same structure as brevo.mdx and documents the exact flow
already in examples/mailtrap — MailtrapClient and send() from mailtrap.

Checked the page renders with mintlify dev; pnpm lint passes and
mintlify broken-links reports no broken links.

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Summary by cubic

Adds the missing Mailtrap integration page so Mailtrap appears in the Integrations nav and overview cards.

• New apps/docs/integrations/mailtrap.mdx page following the same structure as brevo.mdx, documenting the send flow with MailtrapClient and send().
• Registers the page in docs.json and adds a Mailtrap card to the integrations overview snippet.