Add AWS Bedrock LLM Provider Documentation

[jpelletier1]

• I have read and reviewed the documentation changes to the best of my ability.
• If the change is significant, I have run the documentation site locally and confirmed it renders as expected.

Summary of changes

This PR adds documentation for using AWS Bedrock as an LLM provider in OpenHands.

Changes Made

1. Created new documentation file: openhands/usage/llms/aws-bedrock.mdx

   • Explains that OpenHands can work with AWS Bedrock models using an OpenAI-compatible API endpoint
   • Provides step-by-step configuration instructions
   • Includes links to AWS Bedrock's official documentation
   • Contains helpful notes about the openai/ prefix and region configuration

2. Updated navigation: Added AWS Bedrock to the providers list in docs.json

   • Positioned alphabetically after Azure and before Google in the LLM Configuration > Providers section

Documentation Structure

The new documentation follows the established pattern of other provider docs and includes:

• Clear title and description in the frontmatter
• Configuration section with numbered steps
• Example values for Custom Model and Base URL fields
• Two <Note> components to clarify important details about the OpenAI-compatible format and region selection
• Direct link to AWS Bedrock's OpenAI Chat Completions API documentation

The documentation is concise and straightforward, focusing on the essential information users need to configure AWS Bedrock with OpenHands.

@jpelletier1 can click here to continue refining the PR