DOC: consistent import timedeltas docs
Added consistent import for timedeltas docs, i.e.: `import pandas as pd`, `import datetime`.
See: https://github.com/pandas-dev/pandas/issues/9886

Didn't touch the `pandas.tseries.offsets` since no consensus has been made how to import those.
## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14997?src=pr) is 84.78% (diff: 100%)
> Merging [#14997](https://codecov.io/gh/pandas-dev/pandas/pull/14997?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

```diff
@@             master     #14997   diff @@
==========================================
  Files           145        145          
  Lines         51090      51090          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43316      43315     -1   
- Misses         7774       7775     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [eecfa88...f5bd961](https://codecov.io/gh/pandas-dev/pandas/compare/eecfa8843a0c90b5424e9e0e4d353bbb3b78a671...f5bd96190809e6493e373e57fbb1228972ef9255?src=pr) @jackieleng Thanks a lot! 
If you want to do the same for other files, very welcome! (not all have been cleaned up yet)