DOC: add source links to api docs
- [x] closes #14178
- [x] tests not needed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew not needed

cc @jorisvandenbossche  - like you mentioned, this doesn't work for everything (properties, accessors, some generated methods)

Here's an example of how it renders

![image](https://cloud.githubusercontent.com/assets/1924092/18411337/0aa5c680-773b-11e6-8edf-6d77548ce5bb.png)
 looks nice.

@jorisvandenbossche 
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14200?src=pr) is 85.24% (diff: 100%)

> Merging [#14200](https://codecov.io/gh/pydata/pandas/pull/14200?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14200   diff @@
==========================================
  Files           140        140          
  Lines         50563      50563          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43101      43103     +2   
+ Misses         7462       7460     -2   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [6c73e76...67a8161](https://codecov.io/gh/pydata/pandas/compare/6c73e765a5f1191c954c7789791818d2685c3c2c...67a81612647b4819ae0ac42d9b4353047ecc2726?src=pr)
 @chris-b1 Thanks a lot! Then it at least works for those where it is possible
