Disable default markdownlint rules to prevent conflicts with prettier (#50649)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/50649

Changelog: [internal]

`markdownlint` failed on https://github.com/facebook/react-native/actions/runs/14400262967/job/40384250331?fbclid=IwZXh0bgNhZW0CMTEAAR51hcoM4zuGTG4SNbqas4z9X0FsIO8Y9DsCgxJepvD5qSdDezhhIV-uLTpn5w_aem_FF2OiYft3uMzK4xJrKGvuA because lines were too long, but that's already handled by Prettier and I thought that I had disabled that rule in the configuration/

It turns out you have to explicitly set `default: false` to actually disable the default rules and only use the options explicitly set in the config (which this does).

Reviewed By: andrewdacenko

Differential Revision: D72855229

fbshipit-source-id: 8e0c74e8e47e298ebf1450b1a0e31f31179c626c