Launch preferences update: XML-first, .launch extensions, Python launchfile declarative style

[emersonknapp]

Make a fairly opinionated pass on the Launch documentation and tutorials.

Main points:

1. Reduce repetition, use literalinclude to include large blocks especially ones that are repeated in explanations
2. Recommend and apply the .launch extension in all uses except references to actual existing demo files in other packages.
3. Move multilingual launch demos to XML, where possible.
4. By example, use AnyLaunchSubstitution in all Python cases to match XML/YAML usage and move toward a less opinionated default launch usage pattern.

I believe the potentially most contentious part will be the .launch extension, my justification is stated in Launch File Different Formats

While it is not required, we recommend that you name all launch files with the .launch extension, regardless of the language they are written in.

There is historical content that uses the _launch.py, _launch.xml, and _launch.yaml extensions, but this is no longer necessary or recommended as it obfuscates the usage.

Launch files should be considered part of the exposed interface of your package, and your users shouldn't have to know or care which language you used to implement a particular file, just like with your nodes.
Nor should you be tied to keeping it in that format forever!
As long as it has the .launch suffix with an informative name, the caller knows that it is a launch file, and the launch system will autodetect the format as needed.
You can mix and match formats without any issues.

[christophebedard]

Recommend and apply the .launch extension in all uses except references to actual existing demo files in other packages.

Small note:

Python launch files (paths) are only meant be to autocompleted by ros2 launch ... if they end with launch.py. However, it still (probably) gets autocompleted if at least one of the frontends is available, since both the YAML frontend and the XML frontend declare launch as a valid extension. Both frontends are available if the package depends on ros2launch, which is recommended in the "Creating a launch file" tutorial. Again, this is really about autocompletion and not about it working or not.

We could change that if we wanted to, of course.

[emersonknapp]

Converting to draft but leaving open for now. I am going to pull independent pieces out to make incremental progress on the less-dicey bits.