Define rules for major, minor, patch version bumps

[tidoust]

Via #1727 (comment)

Documentation does not say anything about version bumps. It would be useful to clarify and document the rules we follow:

• Major bumps: a backward-incompatible schema, API, or guarantee change.
• Minor bumps: a backward-compatible schema, API, or guarantee change. Also, we currently do a minor bump when a spec gets added/deleted from the resulting package. That latter rule may make less sense in the "consolidated" packages @webref/events and now @webref/css (although if we add back extracts per spec in @webref/css, perhaps we should keep following that rule?).
• Patch bumps: data updates.