ENH: `Styler.bar` extended to allow centering about the mean, value or callable
This refactors `Styler.bar` to allow more flexible arguments to align bars, and an additional keyword `props` akin to other builtin methods that allow some customisation, demonstrated in the new user guide (image below)

- [x] closes #26070
- [x] tests added / passed
- [x] whats new

**All previous tests pass with minor formatting alterations.**

As a follow-on I should improve the original testing for this function - it is sporadic and unstructured.

![Screen Shot 2021-06-29 at 14 53 56](https://user-images.githubusercontent.com/24256554/123800804-f2f42c00-d8e9-11eb-970a-31468d7a29fa.png)
