unify the devcontainer JSON and feature reference regarding mounts

[thebit94dev]

The mounts section in https://containers.dev/implementors/json_reference/
states each value is a (string) matching the Docker CLI --mount flag syntax,
while https://containers.dev/implementors/features/
describes each value as an (object) with the same Docker CLI options.

Kindly update the documentation to unify and clarify that both string and object formats are supported.

Thank you!

[s-h-a-d-o-w]

https://containers.dev/implementors/spec/ also lists it as a string array.

Using this fails validation now, so it looks like the string array has actually been deprecated: https://github.com/s-h-a-d-o-w/devcontainers-features/actions/runs/29635207314/job/88056222161