WEB: Fix extension of about/index
- [✓] closes #29325
- [✓] whatsnew: the about/index.html file is now about/index.md

@datapythonista 
Here is a screenshot of the page that was generated locally. I just noticed that the URL shows the .html extension at the end. Should it be .md or is it ok as shown?

![Screenshot_index_html](https://user-images.githubusercontent.com/56150991/68067532-5e48bd80-fd05-11e9-8112-90ba3ee8c32c.png)
 Thanks, looks perfect now. The extension in the web needs to be html, we convert markdowns to html if they have the right extension.

This is ready, will let someone else from the team review and merged.

Thanks for the fix! lgtm. merge when ready. Sorry for the delay @est271 and thanks a lot for your contribution. Hope to see more coming! @datapythonista No worries. Glad to help.