Add a status indicator for lessons and tutorials

[matentzn]

Pages in OBOOK have a wide variety of quality. I would suggest adding a status flag under the lesson title that gives a sense of how mature a lesson actually is. I would suggest the following "maturity index"

[matentzn]

Done: https://oboacademy.github.io/obook/reference/obook-maturity-indicator/

[shawntanzk]

This is awesome! such a good idea! thanks @matentzn :)

[matentzn]

Thanks :)