docs: add documentation to CONTRIBUTING.md (#8878)

### Description

Adds a new "Skipping CI" section to CONTRIBUTING.md documenting the
native GitHub Actions commit-message mechanism for skipping CI
pipelines.

The section covers:
- Supported keywords (`[skip ci]`, `[ci skip]`, `[no ci]`, `[skip
actions]`, `[actions skip]`)
- The `skip-checks: true` trailer alternative
- Which workflows are affected vs unaffected by the skip instruction
- The caveat about required checks remaining in "Pending" state
- Guidance on when to (and not to) use `[skip ci]`

### Types of changes
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [ ] Breaking change (fix or new feature that would cause existing
functionality to change).
- [ ] New tests added to cover the changes.
- [ ] Integration tests passed locally by running `./runtests.sh -f -u
--net --coverage`.
- [ ] Quick tests passed locally by running `./runtests.sh --quick
--unittests --disttests`.
- [ ] In-line docstrings updated.
- [x] Documentation updated, tested `make html` command in the `docs/`
folder.

---------

Signed-off-by: R. Garcia-Dias <rafaelagd@gmail.com>
Co-authored-by: Eric Kerfoot <17726042+ericspod@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: 3 people

CONTRIBUTING.md

@@ -303,6 +303,13 @@ By making a contribution to this project, I certify that:
     this project or the open source license(s) involved.
 ```
 
+> **Tip:** If you need to add a DCO remediation commit (e.g., after a force-push
+> or rebase), include `[skip ci]` in the commit message so the remediation
+> does not trigger unnecessary CI pipelines:
+> ```bash
+> git commit -s --allow-empty -m 'DCO Remediation Commit for... [skip ci]'
+> ```
+
 #### Utility functions
 
 MONAI provides a set of generic utility functions and frequently used routines.
@@ -358,6 +365,90 @@ Ideally, the new branch should be based on the latest `dev` branch.
 1. Reviewer and contributor may have discussions back and forth until all comments addressed.
 1. Wait for the pull request to be merged.
 
+## Skipping CI
+
+MONAI's CI pipelines run automatically on every push and pull request.
+These pipelines can be resource-intensive, especially the full premerge matrix
+which spans multiple OSes, Python versions, and PyTorch versions.
+
+To reduce unnecessary resource consumption and speed up iteration, you can
+skip CI on commits that don't need automated validation — for example,
+documentation-only changes, README updates, workflow YAML changes, or WIP
+commits during development.
+
+### Mechanism
+
+GitHub Actions natively supports skipping `push` and `pull_request` workflows
+when the commit message contains any of the following strings:
+
+- `[skip ci]`
+- `[ci skip]`
+- `[no ci]`
+- `[skip actions]`
+- `[actions skip]`
+
+These are case-insensitive. `[skip ci]` is the recommended convention for
+this repository.
+
+Alternatively, you can add a `skip-checks: true` trailer at the end of the
+commit message, preceded by two blank lines:
+
+```
+commit message
+
+skip-checks: true
+```
+
+### Usage
+
+Add the keyword anywhere in the commit message when committing:
+
+```bash
+git commit -s -m 'update docs [skip ci]'
+```
+
+If the HEAD commit of a pull request contains the skip instruction,
+the entire PR's pull_request-triggered workflows are skipped.
+
+### Which workflows are affected
+
+The skip instruction applies only to workflows triggered by `on: push` or
+`on: pull_request` events. All other workflows — those using `issue_comment`,
+`repository_dispatch`, `schedule`, or `workflow_dispatch` — use different
+event types and are **not** affected by `[skip ci]`.
+
+### Important caveat
+
+If a workflow is skipped via `[skip ci]`, its associated checks remain in
+"Pending" state. If your pull request requires those checks to pass before
+merging, you will need to push a new commit **without** the skip instruction
+to trigger the CI pipelines.
+
+### When to use
+
+Use `[skip ci]` for commits that are safe to skip CI:
+
+- Documentation-only changes (`docs/`, `README.md`, docstrings)
+- Workflow configuration changes (`.github/`)
+- Repository metadata (`.gitignore`, `CONTRIBUTING.md`, `LICENSE`)
+- WIP or draft commits during local development
+
+Do **not** use `[skip ci]` for commits that change:
+
+- Source code in `monai/`
+- Test files in `tests/`
+- Dependencies (`requirements*.txt`, `setup.cfg`, `setup.py`)
+- Anything that could affect correctness or compatibility
+
+### Quick example
+
+```bash
+git commit -s -m 'fix typo in README [skip ci]'
+```
+
+This commit will be recorded in the repository history but will not
+consume CI minutes.
+
 ## The code reviewing process
 
 ### Reviewing pull requests