Fix Adding commands to a devfile

[themr0c]

In https://docs.devfile.io/devfile/2.0.0/user-guide/adding-commands-to-a-devfile.html

• Add composite in the list of allowed types.
• rename the current example as "an exec command", add an example for a vscode-tasks command.
• reorganize in subpages as for components: Adding exec command, Adding vscode-tasks command, Adding composite command, Adding command in a group.
• previewUrl seems to belong to the exec command description.

[rkratky]

JIRA: https://issues.redhat.com/browse/RHDEVDOCS-2566

[jc-berger]

Are the concerns laid out in this issue still relevant? Especially since we've consolidated the devfile and odo content. The GitHub issue asks for a VS Code example, and we no longer include VS Code related content in this doc.

[jc-berger]

Here's a checklist of what we've done for these concerns:

• Add composite in the list of allowed types.

• rename the current example as "an exec command", add an example for a vscode-tasks command.

• Note: we can rename the current example as "an exec command" and give a similar title to all the examples in the doc. However, we no longer mention vscode-tasks

• reorganize in subpages as for components: Adding exec command, Adding vscode-tasks command, Adding composite command, Adding command in a group.

• Note: do we still want to make subdocs? I want to verify relevance before dividing this doc up.

• previewUrl seems to belong to the exec command description.

[elsony]

• Add composite in the list of allowed types.

Already have info on composite command in the current doc

• rename the current example as "an exec command", add an example for a vscode-tasks command.

We have removed vscode-tasks so this no longer applies. One thing that I note is we still have one reference to the vscode-tasks so we should remove that reference.

• reorganize in subpages as for components: Adding exec command, Adding vscode-tasks command, Adding

composite command, Adding command in a group.
This can still be valid since we currently just have a flat list of steps. It can be organized by adding different sections of the command.

• previewUrl seems to belong to the exec command description.

This is no longer in the spec, no longer applicable