RFC-0043: Rename all rst files to markdown

README.md

rfcs/0001-using-rfcs.md

rfcs/0002-x86-64-v3-microarchitecture.md

rfcs/0003-buildflags.md

rfcs/0004-lto-by-default.md

rfcs/0006-code-of-conduct.md

rfcs/0007-rename-trusted-user-role.md

rfcs/0009-mediation-program.md

rfcs/0010-adopt-pep517-tooling.md

rfcs/0011-store-source-signing-keys.md

rfcs/0014-merge-package-repos.md

rfcs/0016-spdx-license-identifiers.md

rfcs/0017-increase-fortification-level.md

rfcs/0019-aur-moderator-role.md

rfcs/0020-sources-for-python-packaging.md

rfcs/0021-create-a-distro-developer-manual.md

rfcs/0023-pack-relative-relocs.md

rfcs/0025-dbus-broker-default.md

rfcs/0026-fno-omit-frame-pointer.md

rfcs/0029-mirror-definition.md