Update documentation on Component warm_up recommendations

[sjrl]

As a follow up to PR #10181 we should update the docs (e.g. here https://docs.haystack.deepset.ai/docs/components#warming-up-components) to indicate that warm_up is automatically called the first time a component is run.

Users can still explicitly call warm_up themselves, but if they don't the component will warm itself up the first time it's run.