docs: add changelog for 4.0.0-alpha1

[acoulton]

• Write docs page on upgrading, for end-users and extension authors. Highlight changes still planned between alpha1 and 4.0.0
• Write docs page on our new approach to marking the public API since 3.30.0 / docs: Explicitly mark up interfaces, classes & methods in the public API #1807 (see docs: Add and update docs for the 4.0 release docs#228)
• Remove YAML config & annotations from docs in the 4.x branch

[carlos-granados]

Pretty good summary, let's go for this. Thanks @acoulton