Add descriptions to standards on the standards index pages

[baskaufs]

@gkampmeier suggested in #546 (comment) that we add a brief description of each standard to the index page. I agree that this is a great idea and I think that it should be done along with a revision of the standards metadata as served in machine-readable representations.

The machine-readable representations of the standards metadata are generated from this table. When I put the table together, for the description column I just kind of randomly grabbed text from wherever I could find it, often from descriptive content found on a page included in the standard.

So what I would suggest is that we start with what's in this table and revise the descriptions for all of the standards so that they are a similar (short) length and that they include comparable descriptive information. Those same descriptions could then be used on the index pages that we just revised, and perhaps appear in some standardized position on each of the standards landing pages.

This approach would be in keeping with our long-term effort to ensure that the exact same descriptive metadata are provided in the human-readable and machine-readable representations of metadata that we provide.