Create a template notebook for example gallery

[namurphy]

I was thinking that it'd be helpful to have a template Jupyter notebook to simplify the process of adding new notebooks to our gallery.

I'm thinking that the notebook should include...

• An example link to the documentation page on how to create notebooks (currently being developed in Describe how to add example notebooks in the documentation guide #1381)
• A relative link to access a code object in PlasmaPy (like plasmapy.formulary.speeds.Alfven_speed)
• A link to access another notebook in the gallery
• A link to an external website
• A table of contents
• Sample math
• Sample in-line code like `import this`

Also, we would need to...

• Add a link to the example notebook from the contributor guide page on notebook (currently underway in Describe how to add example notebooks in the documentation guide #1381)
• Describe how to make sure the notebook is included in the gallery via the index page
• Make sure that the notebook does not appear in the online gallery (which we should be able to do by putting the template in its own directory that isn't included in the template page)