Improve documentation for MonthEnd and YearBegin offsets
This pull request improves the documentation for two commonly used offset constructors in Pandas: MonthEnd and YearBegin.

Changes include:

Clarified the purpose and behavior of each offset class

Added runnable examples (doctest-compliant) to demonstrate usage

Improved parameter descriptions where necessary

These changes aim to make the documentation more accessible and clear for both new and experienced users of Pandas’ time series functionality.

Looks like something went wrong with this PR, it contains lots of unrelated changes. I think better to close this, as I think it'll be much simpler to open a new PR with the documentation changes than fixing this PR. Please, feel free to reopen for the intended work. I recommend using a new branch (not `main`) to work on pull requests, that should help avoid the problems here.