Add Oracle MAA database skill

[gchawkins]

Adds a new db/maa category for Oracle Maximum Availability Architecture guidance.

Includes:

• Routed MAA overview that directs agents to focused topic files instead of loading a large source catalog first
• Topic guidance for MAA tiers, Data Guard/Active Data Guard, RAC/Exadata, Application Continuity, Oracle Update Advisor, EBR, multicloud DR, backup/recovery, and ZDLRA
• Public Oracle source index for citations and freshness checks
• Oracle Update Advisor REST API and related public resources
• External-safe public Oracle documentation, white papers, and MAA blog references

Validation:

• Scrubbed for internal provenance and non-public support references
• Checked for consistent Oracle Update Advisor naming
• Ran git diff --check
• Commits are signed off