textproc/markdown-mode.el: update to 2.4.

• Switch to use upstream repository on GitHub.
• Byte compile .el file and install them to ${EMACS_VERSION_SITE_LISPDIR}.
• Add DOCS option.

ChangeLog: https://github.com/jrblevin/markdown-mode/blob/v2.4/CHANGES.md

PR: 257899
Reported by: yasu