BLD/DOC: Speed up doc building via jinja2 and conf.py logic

```
~/src/pandas/doc/ λ ./make.py clean ;time ./make.py  ;                      
993.28s
```
```
~/src/pandas/doc/ λ ./make.py clean ;time ./make.py --no-api ;        
255.12s
```
```
~/src/pandas/doc/ λ ./make.py clean ;time ./make.py --single indexing ;                      
8.08s 
```


@jorisvandenbossche , please review.