Enable rumdl markdown line-length enforcement and reflowing (#7667)

This should handle over-long lines. I had tried to make the normalize
method work, but it doesn't seem promising and so let's at least enable
this version.

Assisted-by: Antigravity with Gemini

toolchain/docs/adding_features.md

@@ -512,15 +512,19 @@ file_test infrastructure; see
 There are several supported ways to run Carbon on a given test file. For
 example, with `toolchain/parse/testdata/basics/empty.carbon`:
 
--   `bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/parse/testdata/basics/empty.carbon`
+-   `bazel test //toolchain/testing:file_test
+    --test_arg=--file_tests=toolchain/parse/testdata/basics/empty.carbon`
     -   Executes an individual test.
--   `bazel run //toolchain -- compile --phase=parse --dump-parse-tree toolchain/parse/testdata/basics/empty.carbon`
+-   `bazel run //toolchain -- compile --phase=parse --dump-parse-tree
+    toolchain/parse/testdata/basics/empty.carbon`
     -   Explicitly runs `carbon` with the provided arguments.
--   `bazel-bin/toolchain/carbon compile --phase=parse --dump-parse-tree toolchain/parse/testdata/basics/empty.carbon`
+-   `bazel-bin/toolchain/carbon compile --phase=parse --dump-parse-tree
+    toolchain/parse/testdata/basics/empty.carbon`
     -   Similar to the previous command, but without using `bazel run`. This can
         be useful with a debugger or other tool that needs to directly run the
         binary.
--   `bazel run //toolchain -- -v compile --phase=check toolchain/check/testdata/basics/run.carbon`
+-   `bazel run //toolchain -- -v compile --phase=check
+    toolchain/check/testdata/basics/run.carbon`
     -   Runs using `-v` for verbose log output, and running through the `check`
         phase.