DOC: add new page 'how to read these doc' in get started section
This page explains the code block are run in notebook

- [x] closes #39130
- [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~
- [x] ~whatsnew entry~

Here is a screenshot of the new page: 

![image](https://user-images.githubusercontent.com/15191323/104813161-b46d2800-5807-11eb-93d9-7f174515cd46.png)
 instead of a separate page how about a note at the top of the getting started docs. Thanks for the PR @rhuille but it appears this PR has gotten stale. Please let us know if you have time to merge master and to implement this as suggested in https://github.com/pandas-dev/pandas/pull/39207#issuecomment-779469318 and we'd be happy to reopen. Closing for now. Hi @mroeschke, I've picked up where @rhuille left off and I've added the changes to the top of the Getting Started page.
Does this look better? If so, I'll submit a new PR for my branch and reference it here.

![PandasChanges](https://user-images.githubusercontent.com/20362314/155195157-87be8f83-1e29-4406-8ff5-818bcf419fdc.png) Thanks @ehallam this looks pretty good. One suggestion: Instead of putting this in `Getting Started`, I think it's would be more appropriate in https://pandas.pydata.org/docs/user_guide/index.html since that's where most of the code snippets are. Does this look good @mroeschke? I've also added a "Guides" heading to separate the "How to read these guides" section and the list of guides below.

![PandasChanges4](https://user-images.githubusercontent.com/20362314/155207328-f6714470-d392-4c87-81bf-3952b017997e.png) Looks great! Looking forward to the PR 👍 