New Feature: electron/positron independent source.

[GuySten]

Description

Currently OpenMC already has the capability to simulate electron and positron "transport" using thick target bremsstrahlung (ttb) approximation. This pull request enable the users to use electron / positron source for bremsstrahlung simulation.

This pull request also move the physics description of charged particles to it's own section in the documentation.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)