docs(pro): describe the Support pages, the docs search and the airgapped setting

[svader0]

Adds one Pro documentation page, navigation/PRO__support.md, that describes the Support pages in DefectDojo Pro.

It covers:

• the Support requests page and the Community board
• the documentation search box above the support form, which reads the public docs search index from the instance's server
• how a self-hosted instance enrols with its signed license
• the airgapped setting, which blocks both pages with a dialog and gives the support address
• the three settings, DD_AIRGAPPED, SUPPORT_EMAIL and DOCS_SEARCH_URL, with their defaults

The page uses the same front matter as navigation/PRO__sidebar.md and the Pro-feature notice the other Pro pages use. Built locally with Hugo; it renders at /navigation/pro__support/.