revise README
- point Antora 2 users to antora-lunr project
- add overview of steps to use extension
- shift all section titles down a level
- move installation instructions to separate section
- remove the emoji in front of admonition labels as they won't work when displayed in the npm registry
- retrieve flag emoji from cdn.jsdelivr.net and remove branch reference
- mention the --extension CLI option as an alternative to defining the extension in the playbook file
- put filenames in italic instead of monospace
- remove explicit
generate
command when calling Antora - recommend http-sever instead of serve as the local HTTP server
- rename configuration key from indexLatestOnly to index_latest_only to following Antora convensions
- put title of source example above source block in bold
- fix indentation in YAML files
- add more detailed instructions for how to run tests
- add NTT Application Security to "Who's using it?" section