Install instructions still say 'not in the General registry yet' — revert once #166832 merges

[sotashimozono]

Blocked on JuliaRegistries/General#166832
(New package: ExperimentalAPI v0.1.1, open, three-day waiting period for new packages).

While that is open, the README and docs/src/index.md say:

Not in the General registry yet — install by URL:

pkg> add https://github.com/QAtlasHub/ExperimentalAPI.jl

Once the registration merges that becomes false, and a wrong install line is the first thing a
reader hits. Revert both to:

pkg> add ExperimentalAPI

Same edit in two files:

• README.md, "Install"
• docs/src/index.md, "Install"

Also worth doing in the same PR, since both are only true while unregistered:

• The docs badge points at docs/dev because docs/stable 404s with no tag. Registration
  produces a tag via TagBot, so stable starts resolving and the badge should move back.

Requires a version bump — version / check-version requires exactly one semver step on a human
PR — so this lands as 0.1.2.

Nothing checks this. There is no test that the install instruction matches the registry
state, which is exactly the class of unverified prose this package exists to complain about. If
that irony is worth acting on, it is a separate issue, not this one.