docs: document AWS Bedrock provider

Docs slice of the AWS Bedrock provider work. Updates provider-selection, configuration and api-reference to cover Bedrock auth (API key primary + AWS credential chain optional), region, and cross-region inference profiles.

Umbrella issue (full context): BugTraceAI/BugTraceAI-CLI#8 PR: BugTraceAI/BugTraceAI#16