Documentation: bcachefs: SubmittingPatches: Convert footnotes to reST syntax

Footnotes list are outputted in htmldocs simply as long-running
paragraph instead. Use reST numbered footnotes syntax for the job.

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>