Re-add pyproject.tml to fix unknown env error

[awaismirza92]

1. Fixes Unknown environment: insiders error which occurs during 'Deploy MkDocs documentation' workflow. The error originates from the removal of pyproject.toml as part of PR Bumped version to 0.21.0 #500. This file defines the hatch environment (named insiders) needed to build the docs.
2. Replace unused version key with unused dynamic key. One of them needs to be defined.
3. Add site directory to .gitignore.

[Copilot]

Pull Request Overview

Re-adds pyproject.toml file to fix the "Unknown environment: insiders" error that broke the MkDocs documentation deployment workflow, caused by the file's removal in PR #500. This file defines the hatch environments needed for documentation building.

• Re-added pyproject.toml with hatch environment configurations for default and insiders environments
• Replaced unused version key with dynamic key to comply with packaging specifications
• Added MkDocs dependencies for both environments with insiders using a private repository

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[liuzicheng1987]

Thanks @awaismirza92 !