Document heading class migration

[Mirochill]

Summary

• add an unreleased changelog entry for the heading class structure change
• document how custom CSS selectors should migrate from heading classes to the preceding anchor span
• include an example old/new selector for h2.even

Closes #137

Validation

• git diff --check HEAD~1..HEAD
• checked edited documentation lines stay within 120 characters
• Tests not run locally (documentation-only change).