feat: Overhaul documentation and add advanced usage sections

[oasaph]

Summary

This PR overhauls the documentation and adds advanced usage sections, along with improvements to development and release workflows.

Key Changes by Commit

• Documentation Overhaul & Advanced Usage

  • Added advanced usage docs for custom flag types, dynamic flags, environment/user-based flags, feature rollouts, and best practices.
  • Introduced comprehensive API references for the Flag and Flaggle classes.
  • Created a changelog for documenting notable changes and releases.
  • Enhanced configuration documentation with examples for environment variables and config files.
  • Expanded contributing guidelines to encourage community involvement and maintain code quality.
  • Developed an FAQ addressing common questions and troubleshooting tips.
  • Implemented integration guides for FastAPI, Flask, Django, Typer, and other environments.
  • Added support for MkDocs and various plugins to improve the documentation experience.
  • Included MIT license information.
  • Documented supported operations and JSON schema for the flags endpoint.
    (commit)

• Development Environment

  • Set up a devcontainer with a Dockerfile and configuration for a Python development environment.
    (commit)

• Release Workflow

  • Implemented pre-release versioning and release promotion workflow.
    (commit)

• CI/CD Improvements

  • Changed workflow runners to self-hosted for build jobs.
    (commit)