feat(tvix/docs): switch to mdbook

Some of the docs are still outdated (like architecture and drv path
inconsistencies).

Change-Id: I7a6afceb008ef4cd19a764dd6c637b39fa842a2e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11072
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: edef <edef@edef.eu>

tvix/docs/book.toml

@@ -0,0 +1,11 @@
+[book]
+authors = ["The Tvix Authors"]
+language = "en"
+multilingual = false
+src = "src"
+title = "Tvix Docs"
+
+[preprocessor.plantuml]
+# override the /usr/bin/plantuml default
+plantuml-cmd = "plantuml"
+use-data-uris = true