feat(integrations): add ZeptoMail docs

[Vic-Lara-Gilles]

Adds the missing integration page for ZeptoMail. examples/zeptomail was added in
#3717 but never got a docs page, so ZeptoMail doesn't show up in the Integrations
nav or in the overview cards.

Changes

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

The page follows the same structure as brevo.mdx and documents the exact flow
already in examples/zeptomail — SendMailClient and sendMail() from
zeptomail.

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

Note: this touches the same lines in docs.json and snippets/integrations.mdx
as #3764 (Mailtrap). Whichever merges second will need a trivial rebase; happy
to handle it.

---

Summary by cubic

Adds the missing ZeptoMail integration docs page so ZeptoMail now appears in the Integrations nav and overview cards.

• New apps/docs/integrations/zeptomail.mdx page following the same structure as the Brevo page, documenting the SendMailClient/sendMail() flow already present in examples/zeptomail.
• Registers the page in docs.json and adds a ZeptoMail card to snippets/integrations.mdx.

Note: this touches the same lines in docs.json and snippets/integrations.mdx as the Mailtrap PR; whichever merges second needs a trivial rebase.

^{Written for commit 1194c95. Summary will update on new commits.}

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Latest commit: 1194c95

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