Apply Audience Taxonomy to Development Section Guides

[andystevensname]

Per the Development Housekeeping document, this PR applies the audience taxonomy to the guides in the Development section.

An easy way to test this would be to pull down the Hercules PR that introduces taxonomies, and navigate to docs/audiences. There should only be three of them: Foundational, Beginner, and Intermediate. There were no Expert guides. If there are more than three, then there are probably spelling mistakes in the YAML (though I checked of course and I didn't find any).

[nmelehan]

Rebased after pulling changes from #2185, which incorporates the theme build from
https://github.com/linode/hercules/pull/108

This rebase enables you to see the filled-out audience taxonomy pages; previously, they were empty. I then force-pushed this rebased version history into this PR.

[nmelehan]

This PR changes a large number of guides. These details fulfill our policy on wide-ranging changes in PRs:

1. Include a clear description about what the pull request is trying to achieve and what approach is being taken. The most helpful descriptions guide the reviewer through the code as much as possible, highlighting related files and grouping them into concepts or problems that are being solved.

   1. Audience taxonomy pages were created for docs/audiences, docs/audiences/foundational, docs/audiences/beginner, and docs/audiences/intermediate
   2. The audiences frontmatter was added as an optional tag in our CI rules.
   3. Audience tags from the Development Housekeeping document were applied to each guide in that spreadsheet. These updates make up the bulk of the changes in this PR.

2. A list of repeatable steps that represents all of the changes that were made. In other words, another docs team member should be able to checkout the commit that predates the changes, follow the steps, and end up with exactly all the same changes as in your PR.

   1. To apply the audience tags to the Development guides again, visit the Guide Review tab of the Development Housekeeping spreadsheet. For each guide in that tab, update the corresponding index.md with an audiences frontmatter tag set to the value from the spreadsheet.

3. A set of tests should be included to check the correctness of the changes. These are optimally Travis tests, but might not always be.

   1. Compare the rows of the Guide Review sheet to the contents of each Audience taxonomy page; they should be equal.