Version 2026.9.20, and an install.sh that tells the truth

brand_version! goes to 2026.9.20 (SPEC.md 2.6: YYYY.M.D), the version this
release is tagged at. Verified from the built binary rather than the
source: --version prints "2026.9.20 (Stalwart 0.16.22)" and --help leads
with "INBUXA Server 2026.9.20 (Stalwart 0.16.22)".

install.sh still said "See https://inbuxa.org once it's up", which stopped
being true when the site went up this morning, and offered nothing but a
cargo line. It now names both ways to build, says what a server with no
configuration does, and points at the releases page and the docs.

The installer it stands in for is still unbuilt (SPEC.md 6.1), and the
script says so plainly: an installer trusted with a mail host is not a
thing to improvise, so it declines rather than half-doing one.

README: inbuxa.org is up, so stop saying it is not, and link the docs.

README.md

@@ -11,7 +11,7 @@
 **INBUXA** is a mail and collaboration server: JMAP, IMAP, POP3, SMTP,
 CalDAV, CardDAV and WebDAV, in one Rust binary, with ihasmail as its web front
 end. It is a fork of [Stalwart](https://github.com/stalwartlabs/stalwart).
-Project site: [inbuxa.org](https://inbuxa.org) (not up yet).
+Project site: [inbuxa.org](https://inbuxa.org). Documentation: [docs.inbuxa.org](https://docs.inbuxa.org).
 
 Stalwart ships some features only in a paid Enterprise Edition: multi-tenancy,
 masked email, undelete and others. INBUXA ships everything to everybody under