(WIP) Add migration guide for v2

Based on e71ed5f...c64732c.

Note that 1.2.50, to which this guide refers, doesn't exist yet. We’ll
do that in #1672.

HTML tags used for headings so that I can specify a fixed `id` to use in
fragment links, because I’m probably going to keep editing the headings
and so if I rely on the generated headings then I’ll almost certainly
end up with a broken link. (GitHub doesn’t seem to support the custom
heading ID syntax described in [1].) And we don’t have a linter that
checks for broken fragment links.

Resolves ECO-1416.

(TODO remove) Skipping CI whilst editing this, to avoid loads of
pointless test runs; will undo once editing done:

[skip ci]

[1] https://www.markdownguide.org/extended-syntax/#heading-ids)