Update Contributing Environment section
Closes https://github.com/pandas-dev/pandas/issues/18041
Closes https://github.com/pandas-dev/pandas/issues/17544
Closes https://github.com/pandas-dev/pandas/issues/17747

Still need to

1. Maybe add a section on getting compilers
2. Update the windows environment section
3. More rewording on the docs

But I wanted to get this out there to check if people are OK with the general approach of

1. first conda environment.yaml file with all the build deps
2. second conda requirements file with all the optional deps
3. Instruct contributors to just install everything
4. A script to create requirements.txt files from the conda files for people using virtualenv / pip
5. Just focusing on python3

And if people are able to test this on their machine I'd be grateful. I've tested both conda and pip/venv on my mac.
general approach looks nice.
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18052?src=pr&el=h1) Report
> Merging [#18052](https://codecov.io/gh/pandas-dev/pandas/pull/18052?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/4578a039e95a18b453baf1e6d409b1c4a51aa238?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18052/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18052?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18052      +/-   ##
==========================================
- Coverage   91.24%   91.23%   -0.02%     
==========================================
  Files         163      163              
  Lines       50114    50114              
==========================================
- Hits        45729    45720       -9     
- Misses       4385     4394       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.04% <ø> (ø)` | :arrow_up: |
| #single | `40.24% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18052?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18052?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18052?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.75% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18052?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/18052?src=pr&el=footer). Last update [4578a03...eecb9fc](https://codecov.io/gh/pandas-dev/pandas/pull/18052?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @TomAugspurger awesome! in future PR can add a note in the documentation section of whatsnew