Mark entity types as unstable

[bialpio]

Changes:

• add custom warning about the unstable API
• add required CSS to mark the sections as unstable
• mark sections talking about "entity types" as unstable

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Preview | Diff

[bialpio]

@mounirlamouri @thetuvix @Manishearth - please take a look.

Rendered - the preview above seems to insert its own warning so the custom one added by this PR is not visible.

[bialpio]

Merging now - if you think there are still some issues here, let me know and I'll address them in a follow-up.