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)