doc: add docstrings and examples for String functions

[austinletson]

Add docstrings, usage examples, and doc tests for String.prev, .front, .back, .atEnd.

Improve docstring examples for String.next based on discussion examples for String.prev.

[leanprover-community-mathlib4-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 367b97885a484a2e17e4cdade8dcd7fe05dbfb4c --onto 91244b2dd9d223006227648659203373f5a46b0b. (2024-05-14 22:15:31)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 367b97885a484a2e17e4cdade8dcd7fe05dbfb4c --onto 3035d2f8f689b52963f49b2414414913ca296953. (2024-05-16 02:38:19)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 367b97885a484a2e17e4cdade8dcd7fe05dbfb4c --onto 3a457e6ad60272f03c3555e443379dbde10507ea. (2024-05-18 13:09:42)

[austinletson]

@semorrison Thank you for adding the tests! After all the threads are resolved, I will go back and update the test to reflect the final docstrings.

[austinletson]

I updated the tests based on the updated examples.

Let me know if there is any additional areas for improvement. If not, this should be ready to go!

[austinletson]

Thanks, David! I hope you had a nice holiday!