Consistent use of the "Under Construction" header

[brian-rose]

The issue

There are lots of places in Pythia Foundations where there's a prominent Under Construction disclaimer. These are relics from the early days when we laid out section headings before merging in actual content.

As Foundations reaches young adulthood, I think it's time to review the remaining headers and use them consistently. It's fine to leave them in places where we are clearly planning to develop some new content. But we should clean these up where they no longer serve a purpose.

Places where the header should be removed because the content is mature

• Numpy landing page
• Matplotlib landing page will be mature once we merge the WIP in Matplotlib #230 and Adding matplotlib tutorial content #167
• Xarray landing page
• Pythia Foundations contributor's guide

Places where the header should stay (for now) because we are clearly missing content

• Formatted text in the notebook with Markdown (we need to fill this section in)
• Advanced GitHub Topics ... we can consider this during our review cycle Revisions to all GitHub content #214
• Datetime landing page (we promised a more comprehensive set of lessons here)

Places where I'm not sure whether more content is planned or not:

• Pandas landing page
• Data Formats landing page

[brian-rose]

#272 removes the "Advanced GitHub Topics" page, since we currently have no plans for populating this.

[brian-rose]

#248 addresses the Markdown page

[brian-rose]

It would be great to revisit this during our latest content push prior to AGU 2022.