Add "About compute" (BYOC) operator doc [STORM-3018]

[kris-preza]

Summary

Adds a new Operators Guide page, About compute (amperity_operator/source/compute.rst), documenting Bring Your Own Compute (BYOC) for Databricks. It mirrors the structure and level of detail of the existing About storage (BYOS) page and is wired into the "Build your tenant" grid as "Choose compute".

This is the source draft for STORM-3018 (public documentation), intended to be handed to docs for shaping into docs.amperity.com.

The page covers:

• What BYOC is and what runs on your compute vs. what stays on Amperity-managed compute
• Account prerequisites (storage, Unity Catalog, account admin, region, network/subnet capacity)
• ANSI mode configuration
• The provisioning wizard and re-sync flows
• Debugging
• A shared-responsibility security model
• A manual setup appendix (Unity Catalog objects, SQL, grants)

STORM-3018

Notes for reviewers

• Scope is Databricks only for now; Snowflake BYOC content was removed and can be re-added when the private preview resumes.
• BYOC and BYOS are presented as independent (either can be enabled on its own).
• Manual-setup resource names and grants were verified against the compute-gateway provisioning code so a hand-built setup matches what the service expects.
• Internal IAM role ARNs, account IDs, and bucket/container names are intentionally parametrized as placeholders rather than hardcoded, matching BYOS-doc style.
• The security section reflects the Security Guild review's recommendation to document the shared responsibility model and BYOC boundaries.
• No screenshots are included yet — the page reuses the generic step-number icons. UI screenshots (Add workspace modal, provisioning progress, Stitch "Run on Databricks" selector) still need to be supplied.