PKI-94 Add support for pairing with upstream-CA Sectigo

Greptile Summary

This PR adds documentation for integrating Sectigo as an External CA in Infisical using the existing ACME protocol support, including a new dedicated guide (sectigo.mdx) with step-by-step instructions and screenshots, a navigation entry in docs.json, and a minor update to external-ca.mdx to mention Sectigo in the supported CA list.

The new guide is clear, well-structured, and consistent with the broader PKI documentation patterns. One minor improvement opportunity:

• The example ACME Directory URL (https://acme.sectigo.com/v2/DV) only represents Domain Validated certificates. Sectigo uses different URL paths per certificate profile (OV, EV, IV), and without a clarifying note, enterprise users needing OV or EV certificates could be directed to the wrong endpoint.

Confidence Score: 5/5

• This is a documentation-only PR with no code changes — safe to merge.
• All changes are confined to MDX documentation files, a JSON navigation config, and PNG screenshots. There are no code, API, database, or security-sensitive changes. The single comment flagged is a minor documentation clarity improvement that does not block merging.
• docs/documentation/platform/pki/ca/sectigo.mdx — line 21 should clarify that the DV URL segment varies by Sectigo certificate profile.

Important Files Changed

_{Last reviewed commit: 51f7ad3}

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