DOC: Update CSS alignment for main documentation page
Updates some CSS to better align buttons on the main documentation page. Seems to be an improvement when built locally (I'm using Ubuntu Firefox) but likely worth checking on other platforms / browsers.

Before:
![image](https://user-images.githubusercontent.com/2658661/112676134-0fd61a80-8e36-11eb-863a-9aa16c837a20.png)

After:
![image](https://user-images.githubusercontent.com/2658661/112675603-5ecf8000-8e35-11eb-9a5f-3ad435dba25b.png)
Thanks for looking into this @dsaxton! 
I updated my old PR https://github.com/pandas-dev/pandas/pull/34065, so that is an alternative for this. > Thanks for looking into this @dsaxton!
> I updated my old PR #34065, so that is an alternative for this.

Sure, I was thinking that PR would supersede this one. I mostly put this up in case we wanted to fix the alignment issue only before deciding on whether to use the sphinx extension (we can close this if the other is merged first). Seems Jeff is in favor of the sphinx extension approach so closing this