doc: convert README.org in example to markdown (#7738)

We use markdown for other READMEs, so let's be consistent.

Signed-off-by: Etienne Millon <[email protected]>

example/hello_world.t/README.md

@@ -0,0 +1,16 @@
+This project is called `hello_world`. It defines one library called
+`hello_world` and one executable called `hello_world`.
+
+The library is defined in `lib` and the executable in `bin`. It also defines a
+test in `test`.
+
+At the toplevel of the project, there is a `hello_world.opam` file. This file
+is required so that Dune knows that this is the `hello_world` project.
+
+To build everything that is meant to be installed in this project, type:
+
+    $ dune build @install
+
+To run the tests, type:
+
+    $ dune runtest

example/with-configure-step.t/README.md

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+This project shows how to add a configure step to a project using Dune.
+
+In order to keep things composable, it offers several way to configure
+the project:
+
+1.  with the classic `./configure <args...>`. When doing this, the
+    configuration script are run immediately, and the resulting configuration
+    is frozen for the rest of the build.
+
+2.  by copying `config.defaults` to `config` and editing it. The configuration
+    scripts will be run as part of the build using what is written in `config`.
+    When `config` is edited, the configuration scripts will be automatically
+    rerun.
+
+3.  by doing nothing. The configuration scripts will be run as part of the
+    build using the default values provided in `config.defaults`.
+
+Technically this is how it works:
+
+`configure` is a simple OCaml script that:
+
+- parses the command line
+- generates a `config` file using what was given on the command line
+- calls `ocaml real_configure.ml`
+- `real_configure.ml` does some stuff and generates `config.full`
+
+`config.full` is what is used by the rest of the build.
+
+Now in order to support (2) and (3), we setup the following rules in the
+toplevel `dune` file:
+
+- a rule to produce `config` by copying `config.default`
+- a rule to produce `config.full` by running `real_configure.ml`
+
+This all works as described because if Dune knows how to build a file and this
+file is already present in the source tree, it will always prefer the file
+that's already there.