Make install doc less surprising for new users

[michaelmera]

When reading the docs, after the first tutorial, the "Running Meson" page explains how ninja install installs build targets.
It obviously doesn't work on the tutorial example since there is no target marked for installation. This can be a frustrating experience for new users not aware of Meson default install policy.

This pull request add a remark about tagging targets as installable and a link to the "Installing" doc.

[jpakkane]

Thanks. In the future add [skip ci] at the end of your commit message for doc-only changes. This causes the CI to not run, which saves a lot of time.