Move the "Host your own EPR" docs outside the Fleet reference #9263

Description

What: Find the self-hosted EPR docs a canonical page outside the Fleet reference. Today, the instructions for hosting your own EPR are a section of the Fleet reference page Run Elastic Agents in an air-gapped environment.

Why: Fleet isn't the only thing that uses the EPR. An air-gapped Kibana also needs it for sample data, prebuilt detection rules, and the integrations catalog, and the self-managed, ECE, and ECK air-gapped install guides all tell users to run it. Because the procedure sits in the Fleet docs, deploy-and-manage and Security pages deep-link into a Fleet page to explain a stack-level service. Readers who aren't using Fleet land on a page titled after Elastic Agent, and sections like FIPS 140-3 images get harder to find as the page grows.

Where: We could move that content to a standalone page under deploy-manage that covers hosting the EPR in an air-gapped environment. It could sit next to the self-managed air-gapped install guide or in a shared air-gapped section, since ECE and ECK users need it too.

The issue was flagged during the review of #9212.