Update dimensional analysis for inverse problems

[chaoming0625]

This pull request includes multiple changes across various files to improve unit handling and update documentation examples. The most important changes include the addition of unit handling in several example scripts, the restructuring of documentation, and updates to plotting functions to handle units.

Example Script Updates:

• docs/unit-examples-inverse/brinkman_forchheimer.py: Added unit handling for parameters and variables, ensuring consistent units throughout the script.
• docs/unit-examples-inverse/diffusion_1d_inverse.py: Introduced units for spatial and temporal variables and updated the PDE function to use these units. [1] [2]
• docs/unit-examples-inverse/diffusion_reaction_rate.py: Added units for spatial and concentration variables, and updated the PDE function accordingly. [1] [2] [3]
• docs/unit-examples-inverse/elliptic_inverse_field.py: Incorporated units for spatial, force, and source terms, and updated the solution and training data generation functions. [1] [2]
• docs/unit-examples-inverse/reaction_inverse.py: Added units for spatial, temporal, and concentration variables, and updated boundary and initial condition functions. [1] [2]

Documentation Restructuring:

• docs/unit-examples-inverse.rst: Added a new section for PINN inverse examples, including links to relevant example notebooks.

Plotting Function Update:

• pinnx/utils/external.py: Updated the plot_best_state function to handle units in the plotted data, ensuring that units are displayed correctly in the plot labels.