doc: final round of markdown format changes

Once the README and find-inactive-collaborators.mjs changes land here,
we can introduce markdown formatting as a step in the toolchain
somewhere.

Before this lands, https://github.com/nodejs/node-core-utils/pull/578
will need to land and be included in a node-core-utils release.

PR-URL: https://github.com/nodejs/node/pull/40645


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>