[TASK] Warn that --output is resolved inside the container (#540)

* [TASK] Warn that --output is resolved inside the container

The --output option of the render container is resolved inside the
container: pointing it at a host path outside the mounted volume
writes the files into the container file system, which --rm then
discards - while the command still reports success. Add a note to
the shared local-rendering include so every page showing the
command carries the warning.

Assisted-by: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Sebastian Mendel <github@sebastianmendel.de>

* [TASK] Move the --output warning to where the option is used

Reviewer feedback: the rendering snippet does not use --output, and
_LocalRendering.rst.txt is included in four places, so a note about it sits
in front of readers who never pass the option.

Move it to the Markdown-to-reST howto, the only page in this manual that
passes --output, and shorten it to the point that matters: the path is
resolved inside the container, so it has to stay below the mounted volume.

Assisted-by: Claude Opus 5 <noreply@anthropic.com>
Signed-off-by: Sebastian Mendel <github@sebastianmendel.de>

---------

Signed-off-by: Sebastian Mendel <github@sebastianmendel.de>

Author: CybotTM

Documentation/Howto/Migration/MarkdownToReST.rst

@@ -55,6 +55,13 @@ We will use :file:`Documentation-Migrated` as the output directory.
             New-Item -ItemType Directory -Force -Path ".\Documentation-Migrated"
             docker run --rm --pull always -v ${PWD}:/project -it ghcr.io/typo3-documentation/render-guides:latest --theme=rst --output-format=rst --output Documentation-Migrated
 
+..  note::
+    :bash:`--output` is resolved inside the container, so the path has to stay
+    below the mounted volume (:bash:`-v $(pwd):/project`). A path outside it is
+    written to the container's own file system and disappears with the
+    container, while the command still reports the files as placed. The
+    relative :file:`Documentation-Migrated` above satisfies this.
+
 Now we can copy the :file:`guides.xml` from the original directory documentation directory to the new directory. And remove the
 option `input-format="md"`. This will tell the rendering toolchain to use the ReST files instead of the Markdown files.