DOC: parallelize the number of Sphinx jobs by default
I'm assuming most people building docs would want that to happen as quickly as possible, so this has Sphinx use the maximum parallelism possible by default. [Corresponding Sphinx documentation.](https://www.sphinx-doc.org/en/master/man/sphinx-build.html#cmdoption-sphinx-build-j)

- [ ] ~~closes #xxxx~~
- [x] tests ~~added~~ / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] ~~whatsnew entry~~

does this help? Running on Codespaces, before and after:

```
$ git checkout upstream/master && python make.py clean && time python make.py --no-api
real    5m28.607s
user    5m58.308s
sys     1m50.928s
$ git checkout parallel-default && python make.py clean && time python make.py --no-api
real    3m25.720s
user    6m47.912s
sys     1m44.041s
``` can you merge master and ping on green Anyone know what the `NameError: name 'plt' is not defined` is about? > Anyone know what the `NameError: name 'plt' is not defined` is about?

did you rebase on master? this should have gone away Seems like the problem is still there.

```
$ git rev-parse HEAD
fc9fdba6592bdb5d0d1147ce4d65639acd897565
$ python make.py --single whatsnew/v0.8.0.rst
Running Sphinx v3.3.1
loading translations [en]... done
loading pickled environment... done
[autosummary] generating autosummary for: index.rst
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 1 source files that are out of date
updating environment: [config changed ('exclude_patterns')] 2 added, 0 changed, 1 removed
reading sources... [100%] whatsnew/v0.8.0                                                                    

>>>-------------------------------------------------------------------------
Exception in /root/workspace/pandas/doc/source/whatsnew/v0.8.0.rst at block ending on line 201
Specify :okexcept: as an option in the ipython:: block to suppress this message
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-2-65b822729fc4> in <module>
----> 1 plt.figure()

NameError: name 'plt' is not defined
<<<-------------------------------------------------------------------------
...
``` one more rebase if you pls thanks @afeld 