Default to the stable k3s version instead of v1.19

Changelog for 0.11.1:

Commits
95cf0b1 Set the k3s version to stable by @alexellis
3315af5 Update README.md by @alexellis
6fde5ef Fix two ToC links by @alexellis
337fef3 Update headings and add 2fa note by @alexellis

Generated by Derek