docs: add remaining docs back with some additions/improvements

docs/multi_pipeline.md

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+# Multiple Pipelines
+
+With V2, Nix GitLab CI can generate different pipelines, depending on the
+pipeline source (`$CI_PIPELINE_SOURCE`).
+
+By default, no matter which source, the `default` pipeline is built and ran.
+`$NIX_CI_PIPELINE` can override that, eg. when manually triggering a run.
+To configure which source should be 1-to-1 translated to a pipeline with the
+same name, set `$NIX_CI_DEFAULT_SOURCES` to a regex which explicitly does not
+match these sources. Or set it to an impossible to match regex, then it will
+always run the pipeline named after `$CI_PIPELINE_SOURCE`.
+
+## Example 1: always run default
+
+If you only have a single pipeline, you just have to call it `default`.
+Everything else works out of the box.
+
+## Example 2: default and merge_request_event
+
+If you want the source `merge_request_event` to trigger a different pipeline,
+name it like that and set `$NIX_CI_DEFAULT_SOURCES` to `^(merge_request_event)$`.
+Now a merge request will run this pipeline, while everything else runs `default`.
+
+## Example 3: default, push and web
+
+Set `$NIX_CI_DEFAULT_SOURCES` to `^(push|web)$`.
+
+## Example 4: always run the specific pipelines, never default
+
+Set `$NIX_CI_DEFAULT_SOURCES` to any regex that never matches the sources,
+like `a\A` or `nothing`.