Added links to useful discussions of groupby and SettingWithCopyWarning
Added links to useful discussions of `groupby` and `SettingWithCopyWarning`.

Did not run tests as only docs changed, but `python make.py --single groupby` and `python make.py --single indexing` successfully create the respective html files.

- [x] closes #17505
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21916?src=pr&el=h1) Report
> Merging [#21916](https://codecov.io/gh/pandas-dev/pandas/pull/21916?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2b51c968ca1e16a7fb517968576f8a9ab47ce1ed?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21916/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/21916?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21916   +/-   ##
=======================================
  Coverage   91.96%   91.96%           
=======================================
  Files         166      166           
  Lines       50323    50323           
=======================================
  Hits        46281    46281           
  Misses       4042     4042
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.36% <ø> (ø)` | :arrow_up: |
| #single | `42.23% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21916?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21916?src=pr&el=footer). Last update [2b51c96...927d7c8](https://codecov.io/gh/pandas-dev/pandas/pull/21916?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 worth adding some text / examples from the blog as well?

@datapythonista  I'm unsure about adding many links to third parties in the documentation. If a blog is clearer than the documentation, I think we should improve the documentation as a preferred practice.

@supern8ent what do you think it's missing in the docs that those links clarify?

 @datapythonista Honestly I was at my first sprint and just looking for a doc issue to squash. I agree  that it's better to improve the documentation than link, but I can't answer your question (what specifically is missing) since it wasn't my idea in the first place. 

@jreback I suggest rejecting the PR and clarifying issue 17505 a bit. I thought some is the prose and examples in the blog were pretty clear. might be worth cribbing from it Ok, just including links is not preferred. Better to use those works as a reference to improve pandas documentation.