DOC-3506 - Add firewall whitelist instructions to AI and Doc Converters documentation

[kemister85]

Ticket: DOC-3506

Site:

• CSP page — Firewall and proxy allowlisting (new section)
• TinyMCE AI plugin — Basic setup
• Import from Word plugin — Cloud setup
• Export to Word plugin — Cloud setup
• Export to PDF plugin — Cloud setup
• Cloud deployment of editor & plugins — Forward proxy
• Cloud deployment of plugins only — Forward proxy

Changes:

• Added a new "Firewall and proxy allowlisting" section to the CSP page (tinymce-and-csp.adoc) as a single source of truth, documenting the *.tiny.cloud wildcard domain, individual service subdomains, and required HTTP headers (tiny-api-key, tinymce-api-key).
• Created a reusable admonition partial (admon-cloud-firewall.adoc) for cross-referencing the firewall guidance from plugin pages.
• Added the firewall admonition to the TinyMCE AI (tinymceai.adoc), Import from Word (importword.adoc), Export to Word (exportword.adoc), and Export to PDF (exportpdf.adoc) plugin pages, inside their cloud setup sections.
• Updated the forward proxy configuration sections in editor-and-features.adoc and features-only.adoc to use the *.tiny.cloud wildcard domain instead of listing only three specific service URLs, and added a cross-reference to the CSP page.

Pre-checks:

• Branch is correctly prefixed:
• Files have been included where required (if applicable).
• Build passes without console errors, warnings, or issues.

Review:

• Documentation Team Lead has reviewed.