Add products frontmatter

[colleenmcginnis]

⚠️ This PR is dependent on elastic/docs-builder#1256 being merged and changes being released.

Related to #1336 elastic/docs-builder#1200

Adds new products frontmatter, which will be used to generate metadata during the build process that the web team will use in the search experience (so users can filter by product).

Note: This reduces the scope of #1336 to only include products tags in the frontmatter to unblock updates to the search experience. I'll open a separate PR to update applies_to and tag writers to review for their area.

Here's the process:

• Map AsciiDoc/v3 products: @KOTungseth created a list of all AsciiDoc books mapped to product names.
• Add frontmatter: I wrote a script that uses Kaarina's list to look at each Markdown file and assign products associated with the AsciiDoc book(s) included in mapped_pages.
• Format frontmatter: I standardized the order and format of the frontmatter items.
• Validate frontmatter: I created frontmatter.config.yml and checked against it to make sure all frontmatter keys and product values are valid. (Note: I haven't checked that all the values of deployment and serverless options are valid/correctly formatted.)

cc @KOTungseth @reakaleek @zumwalt

[colleenmcginnis]

This build is going to fail until elastic/docs-builder#1256 is merged and the changes are released.