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

```
~/src/pandas/doc/ λ ./make.py clean ;time ./make.py  ;                      
./make.py --single indexing  993.28s
```
```
~/src/pandas/doc/ λ ./make.py clean ;time ./make.py --single indexing ;                      
./make.py --single indexing  8.08s 
```

@jorisvandenbossche , pleas review