Add blog post introducing the vvbitwarden package

[Copilot]

This PR introduces a comprehensive blog post for the vvbitwarden package, following the established format and style of other package introduction posts in the vusaverse.

Changes Made

• Created blog post (posts/vvbitwarden.qmd) introducing the vvbitwarden package for secure secret management with Bitwarden Secrets Manager
• Added hex logo (assets/hex-vvbitwarden.png) downloaded from the official repository
• Updated packages list (packages.qmd) to include vvbitwarden in the "Packages for external tools" section

Blog Post Content

The blog post includes all essential sections:

• Introduction and package overview
• Prerequisites (BWS CLI installation)
• Installation and authentication setup
• Core functionality with code examples
• Key features and real-world use cases
• Security best practices
• Contributing guidelines
• Links to other vusaverse packages

Code Examples

The post demonstrates practical usage including:

# Basic secret retrieval
api_key <- get_bws_secret("my_api_key")

# Database connection with secure credentials
con <- dbConnect(
  RMySQL::MySQL(),
  password = get_bws_secret("db_password")
)

The blog post follows the same structure and style as existing package introductions (vvcanvas, vvdoctor) and provides comprehensive guidance for users wanting to implement secure secret management in their R workflows.

Fixes #3.

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