ENH: `Styler.apply_index` and `Styler.applymap_index` for conditional formatting of column/index headers
Checks an item on #41648 and #41693

Currently Styler has the `apply` and `applymap` methods to **perform conditional styling on the data elements**.

Recent additions to `Styler.set_table_styles` allows styling of index and column headers if the key-labels are directly specified. But this solution cannot work with LaTeX, nor excel output, and is obviously not conditionally applied.

This PR adds the mechanics to **perform conditional styling on per level index labels**

Follow-ons are:

- [x] updating LaTeX templates to incorporate the styles. (#41993)
- [x] updating Excel output to incorporate the styles. (#41995)
- [x] update Styler user guide

![Screen Shot 2021-06-13 at 10 32 40](https://user-images.githubusercontent.com/24256554/121800538-d3df7400-cc32-11eb-9aaf-8124ba221171.png)
