Fix documentation for power flow models and examples.

[pelesh]

Description

Fixes minor documentation issues in power flow component model and examples README files.

Closes #534

Mentions @shakedregev

Proposed changes

• 1. Remove unnecessary and distracting quotes on math variables here and elsewhere in this part of the docs
• 2. Corrected sentence "In the power system, complex voltage and current values are unknown, but rather real power injections at the generator buses and voltage magnitude setpoint as well as complex power (S) consumed by the load." in the GridKit/Model/PowerFloe/README.md.
• 3. Consolidate using exp instead of Euler's constant
• 4. Replaced $\pi$ with $\Pi$ in the branch model name to avoid confusion with the Ludolph number.

Checklist

• N/A All tests pass.
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows GridKit™ style guidelines.
• N/A There are unit tests for the new code.
• N/A The new code is documented.
• The feature branch is rebased with respect to the target branch.
• N/A The CHANGELOG.md has been updated to reflect the changes. If this is a minor PR that is part of a larger fix already included in the file, state so.

Further comments