Feat/3094 patterns section

[evwilkin]

Demonstrate possible approach to update #3183

1. Update Core & React to add new patterns folders and fill them as needed.
   • In the Core repo create a new patterns directory and move any relevant files out of demos and into patterns.
   • In the React repo create new "patterns" directory and move relevant files out of demos into patterns.
   • Update the frontmatter of all moved .md files to include section: patterns
2. Update patternfly-org/v4's source file to:
   • Edit the existing Core sourceMD command to also exclude the new patterns directory:
     • sourceMD(path.join(coreDocsPath, '!(demos|patterns)/**/examples/**/*.md'), 'html', coreDocsIgnore);
   • Add a new Core sourceMD command to process all the files in the new patterns directory and put them within an "HTML demos" tab:
     • sourceMD(path.join(coreDocsPath, 'patterns/**/*.md'), 'html-demos', coreDocsIgnore);
   • Add a new React sourceMD command to process all the files in the new patterns directory and put them within an "React demos" tab:
     • sourceMD(path.join(reactCorePath, '/patterns/**/*.md'), 'react-demos');

[patternfly-build]

PF4 preview: https://patternfly-org-pr-3196-v4.surge.sh/v4