docs: Add high availability documentation for local HA configuration

[hossain-rayhan]

Summary

Adds comprehensive high availability documentation covering local HA concepts, configuration, and automatic failover behavior.

Changes

New Files

• docs/operator-public-documentation/preview/high-availability/overview.md - HA concepts overview
• docs/operator-public-documentation/preview/high-availability/local-ha.md - Local HA configuration guide

Updated Files

• mkdocs.yml - Added High Availability navigation section and enabled Mermaid diagram support

Documentation Highlights

HA Overview:

• Explains three HA types: Local HA, Multi-Region, Multi-Cloud
• RTO/RPO concepts and trade-offs
• Decision tree for selecting appropriate HA strategy
• Architecture diagrams using Mermaid

Local HA Guide:

• Instance configuration (instancesPerNode: 1-3)
• Pod anti-affinity setup (zone-level and node-level distribution)
• Automatic failover process with CNPG timing parameters
• Documents configurable vs default parameters (stopDelay is currently the only configurable timeout)
• Testing procedures and troubleshooting

Notes

• Failover timing parameters are inherited from CloudNative-PG
• Manual failover for local HA is not currently supported via kubectl plugin (plugin is for multi-cluster promotion only)
• Connection string can be retrieved from kubectl get documentdb <name> -o jsonpath='{.status.connectionString}'