CI (Buildkite): Update all rootfs images to version v3.1, add some documentation, and reorganize/rename/move a bunch of configuration files (#41698)

(cherry picked from commit e6e79f7)

Author: DilumAluthge

.buildkite/README.md

@@ -0,0 +1,5 @@
+# Buildkite
+
+This directory contains the Buildkite configuration files for Base Julia CI.
+
+The rootfs image definitions are located in the [rootfs-images](https://github.com/JuliaCI/rootfs-images) repository.

.buildkite/coverage-linux64/0_webui.yml

@@ -15,4 +15,10 @@ steps:
           # but some of our signed pipelines can wait upon the completion of these unsigned
           # pipelines.
           unsigned_pipelines:
-            - .buildkite/experimental/pipeline.yml
+            - .buildkite/pipelines/experimental/launch_unsigned_builders.yml
+
+          # Our signed pipelines must have a `signature` or `signature_file` parameter that
+          # verifies the treehash of the pipeline itself and the inputs listed in `inputs`
+          # signed_pipelines:
+          #   - pipeline: .buildkite/pipelines/experimental/misc/foo_bar_baz.yml
+          #     signature: "my_signature"

.buildkite/coverage-linux64/README.md

@@ -0,0 +1,7 @@
+## Experimental pipeline (`master` branch only)
+
+This is the [`julia-master->experimental`](https://buildkite.com/julialang/julia-master-experimental) pipeline.
+
+We use this pipeline for builders that are not yet stable enough to go into the main pipeline.
+
+These builders are triggered by GitHub webhook events, such as pushes and pull requests.