Create set of metadata for <info>

[tomschr]

Problem Description

We've discussed several ways how we could deal with metadata. Some ideas are in the article DocBook and metadata. cf also jsc#DOCTEAM-238.

However, none of them was really appropriate, so we need to define our own set.

Design Ideas

• Define our own set in a different namespace.
• Should be easy to write and memorize.
  • Ease of manual use can help avoid the need for a custom tagging tool that may soon end up unmaintained (like docmanager nowadays)
• Should be as validatable as possible: it should be possible to validate concrete metadata types and in some cases also content (e.g. for stuff like dates), rather than just being able to validate a generic tag name.
• Extensibility should be provided by quick additions to the schema rather than by being able to define new tags ad-hoc
  • Think about whether it makes most sense to ship within GeekoDoc or ship separately, so it's not bound to GeekoDoc releases
• Should be XML-based, for use with stylesheets
  • Possible goal: make it usable from native AsciiDoc to avoid the necessity of docinfo file
• Possible goal: Custom tags should allow for straightforward conversion into a standard tag set such as DocBook's <meta/> tags or RDFa Lite.
• Possible goal: Include functionality from dm: namespace/supersede dm: namespace.

Related issues/PRs

• multiple dates: <info/> can contain multiple <date/> tags #74
• productname: Make it possible to define multiple products for the same document #87
  • there are two use cases here that should probably be separated:
    • profiling that can be switched on/off directly on the page, depending on the productname
    • documents that document a combination of multiple products
    • (combinations of both are probably relevant as well.)
• translation tags: Implement a technical solution to track translations on a file base daps#596
• length-restricted SEO description tags for HTML content: Open Graph, Twitter Cards, <meta/> tags - Shorten Open Graph title/desc & meta desc doc-suse-com/site#52
• [may be out of scope] personname: Allow othername & lineage #83 & Allow personname related elements #91

[initial description by toms, large parts of bottom section of issue by sknorr]