Bazelify the jekyll handling. (#230)

- Shift some things around to adjust to being in bazel.
- Add separate build/serve/publish scripts for use by bazel (not set up for direct execution, but bazel requires +x).
- Fix some tests I noticed not running unittest.main as a result of the switch.
- Move md files into filegroups for build reuse.
- Disable automatic site publishing (now `bazel run //website/jekyll:publish`)

Site publishing seems like it'd be too much trouble to keep automated... The C++ toolchain essentially needs to be set up due to the repo config, along with syncing the LLVM submodules, etc. That seems a bit annoying to do on each run of the publish-docs action, and not something I really want to maintain. If we get a CI, we can focus on it more there, but this feels like it'd just be a one-off to maintain as a github workflow.

github_tools/BUILD

@@ -12,30 +12,35 @@ py_library(
 py_test(
     name = "github_helpers_test",
     srcs = ["github_helpers_test.py"],
+    python_version = "PY3",
     deps = [":github_helpers"],
 )
 
 py_binary(
     name = "pr_comments",
     srcs = ["pr_comments.py"],
+    python_version = "PY3",
     deps = ["github_helpers"],
 )
 
 py_test(
     name = "pr_comments_test",
     srcs = ["pr_comments_test.py"],
+    python_version = "PY3",
     deps = [":pr_comments"],
 )
 
 py_binary(
     name = "update_label_access",
     srcs = ["update_label_access.py"],
+    python_version = "PY3",
     deps = ["github_helpers"],
 )
 
 py_test(
     name = "update_label_access_test",
     srcs = ["update_label_access_test.py"],
+    python_version = "PY3",
     deps = [
         ":pr_comments",
         ":update_label_access",