Fix how-to-vex file extenstion to .md for Markdown; fix typos

[bact]

• A follow-up of Create how-to-vex.mc #6
• Rename how-to-vex.mc -> how-to-vex.md, so it can be rendered
• Fix few typos: "metdata" -> "metadata", "logj4" -> "log4j", etc.
• Fix Markdown warnings: add one blank lines around headings and lists
• Remove "J" numbering from the heading, as it is no longer an appendix

[bact]

Please kindly review @rnjudge - thank you. (see list of changes above)