[docs] Fix documentations

[Routhleck]

This pull request includes several updates to the documentation, including the addition of a new Makefile for Sphinx documentation, a comprehensive overview of the PINNx library, and restructuring of example files. The most important changes include the creation of a minimal makefile for Sphinx, a detailed description of PINNx's features and algorithms, and the reorganization of example documentation.

Documentation improvements:

• docs/Makefile: Added a minimal makefile for Sphinx documentation, allowing easier management and building of documentation.
• docs/about.rst: Added a detailed description of PINNx, including its features, algorithms, and integration with the Brain Dynamics Programming ecosystem.

Reorganization of example documentation:

• docs/examples-unit.rst: Added examples for PINN with Unit, including various forward problem examples.
• docs/examples-unitless.rst: Added examples for PINN with Unitless, featuring multiple forward problem scenarios.

Removal of outdated example documentation:

• docs/examples-function.rst, docs/examples-operator.rst, docs/examples-pinn-forward.rst, docs/examples-pinn-inverse.rst: Removed outdated example documentation to streamline and update the examples provided. [1] [2] [3] [4]

Index restructuring:

• docs/index.rst: Updated the index to include new sections for installation, quick start, user guide, and about PINNx, and removed the old example references.