Tweak `markdownlint` rules and README versioning #772

Job	Run time
Environment variables (25, v1.14, 1.0.0, nightly, windows-latest)	44s
OTP 27, Elixir v1.17.0, Gleam , rebar3 3.23	1m 36s
OTP 26.0, Elixir main, Gleam , rebar3	59s
OTP 26.0, Elixir v1.14-otp-25, Gleam , rebar3	44s
OTP 26.0, Elixir 1.14.5, Gleam , rebar3	55s
OTP 25, Elixir , Gleam , rebar3 nightly	1m 34s
OTP 24.0.2, Elixir , Gleam , rebar3 3.16	1m 44s
OTP 26.2.5, Elixir , Gleam , rebar3 nightly	1m 48s
OTP 23.0, Elixir , Gleam , rebar3 3.15	1m 33s
OTP 23, Elixir v1.10, Gleam , rebar3 3.14	1m 0s
OTP 24, Elixir v1.13, Gleam , rebar3 3.15	1m 49s
OTP 24, Elixir , Gleam 0.19.0-rc3, rebar3	33s
OTP 24, Elixir , Gleam 0.23.0-rc1, rebar3	31s
OTP 24, Elixir v1.13, Gleam , rebar3	45s
	16m 15s

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