Set up for 8.1.0 previews

Also fix name of the main branch in Antora, no need to introduce a
confusing `next` seen nowhere else.

Author: brunchboy

.github/resources/preview_notes.md

@@ -3,6 +3,8 @@
 Although for packaged products you may hesitate about using “unreleased” code, for an experimental project like Beat Link Trigger, this is probably your best bet. Releases are only frozen when things are slow, and fixes are never back-ported to older releases.
 
 > [!TIP]
+> You can jump directly to the branch of the [online user guide that matches this preview version](https://blt-guide.deepsymmetry.org/beat-link-trigger/main/README.html].
+>
 > Really, the main disadvantage of using the preview release is that you need to remember to come back here and get an updated copy every so often, to keep up as new things are added. You can compare your build date in the About box with what you find here.
 
 Ideally you should also [join the conversation on Zulip](https://deep-symmetry.zulipchat.com/#narrow/channel/275322-beat-link-trigger) so you will be alerted to fun new things as soon as they are ready to test, and it is easy to give feedback and help set the next goals.

doc/antora-assembler.yml

@@ -1,4 +1,4 @@
 build:
   command: bundle exec asciidoctor-pdf -a source-highlighter=rouge --theme default-with-font-fallbacks
 component_version_filter:
-  names: 'next@*'
+  names: 'main@*'

doc/antora.yml

@@ -1,7 +1,6 @@
 name: beat-link-trigger
 title: Beat Link Trigger User Guide
-version: 'next'
-display_version: 'main'
+version: 'main'
 prerelease: true
 start_page: ROOT:README.adoc
 asciidoc: