Update docs

[chaoming0625]

This pull request includes updates to the documentation and examples for the PINNx library, focusing on adding detailed examples and improving the clarity of the documentation. The most important changes include adding a new quickstart example, updating the installation section, and renaming example files for consistency.

Documentation enhancements:

• README.md: Added a detailed quickstart example demonstrating how to define a PINN with explicit variables and physical units.
• docs/index.rst: Updated the installation section with a new quickstart example, providing step-by-step instructions for defining and solving a PINN problem.

Content reorganization:

• docs/index.rst: Updated the user guide to reference the correct example file.
• docs/unit-examples-forward.rst: Renamed example file Burgers.ipynb to burgers.ipynb for consistency.

General improvements:

• docs/index.rst: Enhanced the description of PINNx to include information about its integration with the BDP ecosystem and its capabilities for defining problems with explicit variables and physical units.