Add a wildfly-design section to wildfly.github.io

The https://github.com/wildfly/wildfly.github.io is the git repo that backs the docs.wildfly.org site, which is published via GitHub Pages. Add a wildfly-design section to it to house ad-hoc, non-version specific design documentation.

The wildfly-design folder should include an index.adoc file and one or more toc-<topic_name>.adoc documents. The wildfly-design/index.adoc should consist of introductory material explaining the purpose of the design docs section and links to the top level toc-<topic_name>.adoc documents. A toc-<topic_name>.adoc file should have a brief explanation of the concept of the topic, and then links to specific documents. Specific design docs should be linked from an appropriate toc-<topic_name>.adoc file (and perhaps to other toc-<topic_name>.adoc files if appropriate.)

A flat structure should be maintained, in order to improve the likelihood of stable URLs for documents. Attempts at organizing via subfolders will likely lead to future reorganization and broken links.

At this point no fancy behavior or conversion of content to html, beyond what GitHub pages already does for the site as a whole, is in scope. The purpose of this section is to preserve for posterity the design thinking behind various aspects of WildFly.

There is no intention of ever updating this content in association with any WildFly releases. It's certainly fine for specific docs to be living documents that are updated as their authors see fit, but there should be no expectation for maintenance of published docs.

My main purpose with this JIRA is to follow the KISS principle and get something going as we've been talking about it for years.