Update handling of missing kernel headers to better reflect warning status

[ddelnano]

Summary: Update handling of missing kernel headers to better reflect warning status

The px cli was updated to include a warning when it detected that a cluster didn't have kernel headers installed. The initial implementation reused the same UI elements that the cli included and therefore the "warning" was printed with red text prefixed with ERR: (as seen below).

This was harsher than intended and could cause confusion for users that otherwise have a working cluster. This PR updates the warning with yellow text and a more appropriate prefix (WARN: ) and links to documentation (added in https://github.com/pixie-io/px.dev/pull/111) that provides more detail.

Relevant Issues: #2051

Type of change: /kind bugfix

Test Plan: Deployed a new vizier that failed the check and verified the text is no longer red and links to documentation