Improve documentation navigation / dataset doc content

[yymao]

This PR improves the documentation navigation by

• Adding the doc menu as a left sidebar on documentation pages
• Adding links to dataset docs on "Transfer" and "Browse Datasets" pages

This PR improves the dataset documentation content by

• Adding a one-sentence summary of the dataset nature at the very beginning
• Mentioning the original format, where to find information about partition and data schema.
• Mentioning the potential issue of parquet on old machines
• Mentioning that, for cosmoDC2, the raw files may not contain the quantities in the schema.

We should test this PR on the dev site to make sure the documentation navigation works correctly, and make any style adjustments as needed.

[heather999]

Deploying to the dev site for a look.

[heather999]

The browse dataset pages currently cause an Internal Server Error: http://lb.desc-web-dev.production.svc.spin.nersc.org/browse/dataset/11767c28-e6c8-4901-92de-3e3d7502c7d4

Looking at the logs:

  File "/app/portal/templates/browse.jinja2", line 17, in block "body"

    <a href="{{url_for('render_doc', doc_name=dataset['doc_name'])}}">Learn more about this data set.</a>

  File "/usr/local/lib/python3.8/dist-packages/jinja2/environment.py", line 452, in getitem

    return obj[argument]

jinja2.exceptions.UndefinedError: 'dataset' is undefined

128.55.137.182 - - [14/Jan/2021 14:10:13] "GET /browse/dataset/11767c28-e6c8-4901-92de-3e3d7502c7d4 HTTP/1.1" 500 -