[3] Md app url rename

[nkoenig]

I changed 'app.ignitionrobotics.org' to 'app.gazebosim.org' in the markdown files using:

find . -name "*.md" -exec sed -i 's/app\.ignitionrobotics\.org/app\.gazebosim\.org/g' {} \;

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.