docs: platform v12 release blog

[KaiVandivier]

Part of LIBS-689

Mostly copied from the head of dhis2/app-platform#884

To do:

• Get a proper link to the migration guide
• Update the sidebars (for the app platform, both the migrate js-to-jsx script and migration guide)
• Test 'diff' code block rendering [fixed by instructions here]
• Uncomment links once app-platform PR is merged
• Update blog date before releasing

[Topener]

Great blogpost, thanks for putting in the work! Just a few minor improvements. Not sure if the (commented out) links already worked, but making them absolute is more of a standard in use. Let me know your thoughts on the comments.