Notes that `GIT_STRATEGY` should be set to `none` so that the
job doesn't fail when triggered automatically when the branch
is deleted.

The variable $GIT_CLONE_PATH does not support nested variables, it will
only expand the variables once, not more.

reference https://gitlab.com/gitlab-org/gitlab-runner/issues/4319

We frequently see new issues mentioning that something defined
in `before_script` or `script` doesn't work with `after_script`.

Our documentation mentions that first two are concatenated and
executed together, why the third one is executed in a separated
shell context. But it looks like it's not clear enough.

Since this misunderstanding of how scripts are being executed
is repeated frequently, let's make it more clear in docs.

Fix more links after review

Update redirect_from link

Apply suggestion to doc/ci/introduction/index.md

Apply suggestion to doc/ci/README.md

Apply suggestion to doc/ci/README.md

Make front matter redirect_from links absolute

Change front matter to fix discus comments

TBA to a different MR/issue

as requested by https://gitlab.com/gitlab-org/gitlab-ce/issues/30477#note_29060568 in #30477

Even though there's no relation to the feature (it's actually about
overriding template values, best to replace this with
$KUBE_INGRESS_BASE_DOMAIN to avoid any potential confusion

Related to gitlab-ee/!5566

no leading slash at project reference

This adds a limitation that we will try to create pipeline
for at most 4 first changes (branches and tags).

This does not affect processing of Pipelines for Merge Requests,
as each updated MR will have associated pipeline created.

CI ref pattern matching now uses RE2 instead of Ruby.
See
https://about.gitlab.com/2019/04/01/security-release-gitlab-11-dot-9-dot-4-released/ for more information.

This adds documentation for:

- `CI_BUILDS_DIR`,
- `CI_CONCURRENT_ID`,
- `CI_CONCURRENT_PROJECT_ID`,
- `GIT_CLONE_PATH`,
- `GIT_CLEAN_FLAGS`

This brings support for untrusted regexp for 'only:refs:' when
enabled via feature flag: alllow_unsafe_ruby_regexp.

This is by default disabled, and should not be used in production

At-symbols are used to match a ref to a specific repository path.
Currently this is done by string splitting the ref pattern on the
at-symbol. In order to pattern match a literal `@` you must use
ruby's unicode character matching `\u{0040}`.

Using we and you isn't very pleasant to read nor is it professional. So
lets remove these invocations to have a more pleasant read.
Signed-off-by: NOlliver Schinagl <oliver@schinagl.nl>

This makes ref validation to use always `UntrustedRegexp`.
This also splits the existing RubySyntax into separate
class.

The 'master' branch name is confusing as it indicates some special branch name,
where it is actually just a 'ref'. 'branches' _is_ a special name as it
indicates 'all' branches.

Lets clarify the example a little to indicate this.
Signed-off-by: NOlliver Schinagl <oliver@schinagl.nl>