DOC: Group optional dependencies by category
- [x] closes #39594 
- [ ] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

This PR splits the Optional Dependencies table into multiple tables one for each category. It should help people compare different options when looking for a specific capability, e.g. "I need to read an Excel file. What are my options?".

The specific grouping felt pretty subjective. If you think of a better way to group them, we can do that instead.
looks fine. can you post a rendered screenshot? @jreback 

Here's a [pastebin](https://pastebin.com/EJtracLm) of the full html page. And here's a couple screenshots.

![1](https://user-images.githubusercontent.com/35386821/107526903-8d3f1780-6b75-11eb-98bb-7cb1b883928b.png)
![2](https://user-images.githubusercontent.com/35386821/107526907-8e704480-6b75-11eb-8f48-e80010942f2f.png)
![3](https://user-images.githubusercontent.com/35386821/107526912-8f08db00-6b75-11eb-928b-4f1aab2e6cef.png)
 @nmay231 thanks, yeah looks good. ping after @jorisvandenbossche suggestions are pushed & green. @jorisvandenbossche can you merge master @jreback Merged with master @jreback Updated @jreback Sorry for the misunderstanding. It's fixed now. thanks @nmay231 very nice.

https://pandas.pydata.org/pandas-docs/dev/ will have the built docs (once CI is done), check that everything renders properly.

thanks!