Document how to compile a reproducible build (#18953)

* Make compiler build more reproducible

Removed the compile time from the version output.
Added Git commit's author's datetime in UTC timezone.

Fixes #18508

See https://reproducible-builds.org/docs/timestamps/
See https://reproducible-builds.org/docs/source-date-epoch/

* Revert "Make compiler build more reproducible"

This reverts commit 2f9359ae89622c2d95ef243d13251a310e94a199.

Documented how to use compile reproducible builds with the
`SOURCE_DATE_EPOCH` environment variable.

* Corrected `readme.md` formatting

Co-authored-by: quantimnot <quantimnot@users.noreply.github.com>

doc/intern.rst

@@ -68,6 +68,17 @@ More information about its options can be found in the `koch <koch.html>`_
 documentation.
 
 
+Reproducible builds
+-------------------
+
+Set the compilation timestamp with the `SOURCE_DATE_EPOCH` environment variable.
+
+.. code:: cmd
+
+  export SOURCE_DATE_EPOCH=$(git log -n 1 --format=%at)
+  koch boot # or `./build_all.sh`
+
+
 Developing the compiler
 =======================