Remove docker and devcontainer as unused (#5500)

As far as I'm aware, the the devcontainers aren't in frequent use, which
is why they fall out of date. Comparing with
https://github.com/llvm/llvm-project/, I don't see devcontainer configs
maintained as part of llvm
(https://github.com/llvm/llvm-project/issues?q=devcontainer doesn't have
much either) so I think we should trim these instead of investing in
maintenance.

These configs aren't being maintained. In the docker configs, note `RUN
bazel build //explorer` is broken. Also, #5496 noted the clang version
is out of date.

Closes #5496

docs/project/contribution_tools.md

@@ -227,13 +227,6 @@ considering if they fit your workflow.
         **either** normal terminals **or** Visual Studio Code to run `bazel`,
         not both in combination. Visual Studio Code can still be used for other
         purposes, such as editing files, without interfering with `bazel`.
-    -   [DevContainers](https://code.visualstudio.com/docs/remote/containers): A
-        way to use Docker for build environments.
-        -   After following the
-            [installation instructions](https://code.visualstudio.com/docs/remote/containers#_installation),
-            you should be prompted to use Carbon's
-            [devcontainer](/.devcontainer/devcontainer.json) with "Reopen in
-            container".
 -   [clangd](https://clangd.llvm.org/installation): An LSP server implementation
     for C/C++.
     -   To ensure that `clangd` reports accurate diagnostics. It needs a