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Docs: Links documentation to working example for CMD Shell usage on Windows GitLab Runners where only PowerShell can be the default

Related to: gitlab#233529

Related to: technical-writing#337

Related to: &5020

Some docs pages were missing metadata; this commit adds the metadata.

Revert removal of Windows Batch support

See merge request !2112

This reverts commit 8bcfc438, reversing
changes made to e54050ea.

The `BuildStagePrepare` is used to echo the hostname of the machine that
is running the job. If this fails we mark it as a BuildFailure which is
not the case, it should be a system failure. If we fail to prepare the
environment it's the Runner fault and not the users fault.

Since the failure is marked as system failure we no longer try to run
the [artifact upload on
failure](https://gitlab.com/gitlab-org/gitlab-runner/-/blob/73ac97aa362bf5dc4d4ea4425d098974a8d309b5/common/build.go#L317)
since the job didn't run and there is nothing to upload.

Having this marked as a system failure would make it more clear to the
user that the problem is with the Runner environment since the user
would not see the "No such directory" error from the `cd` we try to
execute on `BuildStageUploadOnFailureArtifacts`.

Add a link to the documentation to make it more simple for them to
understand what might be going on.

reference #1379

It seems a bit strange having a section with just a tip, change it to be
a normal paragraph.

Removes the bash codeblock tags and changes
them to shell, toml, batch, etc.

In !1563 we
changed the actual script we use to generate the directory for Windows.
Updated the example to reflect this.

It was suggested in
!1406 (comment 190409847)
that it's not very clear when PowerShell is the default shell and when
Batch is the default shell.

Since !1260 we
used delayed expansion for errorlevel when running Windows Batch.

Rename `shells/README.md` to `shells/index.md` to follow the new naming
convention
https://docs.gitlab.com/ee/development/documentation/styleguide.html#working-with-directories-and-files

closes #4181

Applying wording changes from a review for !1209

Internally, the runner uses paths with the `/` separator, but
since CI_PROJECT_DIR is an externally-facing path that can be used in
scripts, we need to ensure that it has the right separator.

Closes #2841