Update contributing docs and PR template for changelog fragments

The fragment-based changelog system landed in isaac-sim#5434. Two contributor-
facing references still pointed at the old "edit CHANGELOG.rst directly"
workflow:

- docs/source/refs/contributing.rst — Maintaining a changelog and
  extension.toml section described per-version editing of CHANGELOG.rst
  with manual SemVer bumps.
- .github/PULL_REQUEST_TEMPLATE.md — checklist item asked contributors
  to update the changelog and bump extension.toml.

Replaced the directly-affected paragraphs and the sample with the
fragment workflow. Section/style guidance (Added/Changed/Deprecated/
Removed/Fixed, past tense, sample bullets) stays verbatim.

Author: hujc7

.github/PULL_REQUEST_TEMPLATE.md

@@ -47,7 +47,7 @@ To upload images to a PR -- simply drag and drop an image while in edit mode and
 - [ ] I have made corresponding changes to the documentation
 - [ ] My changes generate no new warnings
 - [ ] I have added tests that prove my fix is effective or that my feature works
-- [ ] I have updated the changelog and the corresponding version in the extension's `config/extension.toml` file
+- [ ] I have added a changelog fragment under `source/<pkg>/changelog.d/` for every touched package (do **not** edit `CHANGELOG.rst` or bump `extension.toml` — CI handles that)
 - [ ] I have added my name to the `CONTRIBUTORS.md` or my name already exists there
 
 <!--

docs/source/refs/contributing.rst

@@ -149,20 +149,28 @@ for each version of the extension.
 
 .. note::
 
-   The version number on the ``extension.toml`` file should be updated according to
-   `Semantic Versioning <https://semver.org/>`__ and should match the version number in the
-   ``CHANGELOG.rst`` file.
+   ``CHANGELOG.rst`` and ``extension.toml`` are compiled by CI from per-PR **fragment
+   files** — contributors do not edit them directly. For every package your PR touches
+   in ``source/<package>/`` (outside ``changelog.d/``), add one fragment under
+   ``source/<package>/changelog.d/<slug>.<tier>.rst``:
+
+   * ``<slug>.rst`` — patch bump
+   * ``<slug>.minor.rst`` — minor bump (new public API)
+   * ``<slug>.major.rst`` — major bump (breaking change)
+   * ``<slug>.skip`` — no entry, no bump (CI / docs / test-only PRs)
+
+   ``<slug>`` is any short, unique name; your branch name with ``/`` replaced by ``-``
+   is the recommended default. Within a batch the highest tier wins for the package.
+   The version on ``extension.toml`` is bumped by CI according to
+   `Semantic Versioning <https://semver.org/>`__.
 
 The changelog file is written in `reStructuredText <https://docutils.sourceforge.io/rst.html>`__ format.
 The goal of this changelog is to help users and contributors see precisely what notable changes have
 been made between each release (or version) of the extension. This is a *MUST* for every extension.
 
-For updating the changelog, please follow the following guidelines:
+For each fragment, please follow the following guidelines:
 
-* Each version should have a section with the version number and the release date.
-* The version number is updated according to `Semantic Versioning <https://semver.org/>`__. The
-  release date is the date on which the version is released.
-* Each version is divided into subsections based on the type of changes made.
+* Each fragment is divided into subsections based on the type of changes made.
 
   * ``Added``: For new features.
   * ``Changed``: For changes in existing functionality.
@@ -182,16 +190,10 @@ For updating the changelog, please follow the following guidelines:
 
    When in doubt, please check the style in the existing changelog files and follow the same style.
 
-For example, the following is a sample changelog:
+For example, ``source/isaaclab/changelog.d/<slug>.minor.rst``:
 
 .. code:: rst
 
-    Changelog
-    ---------
-
-    0.1.0 (2021-02-01)
-    ~~~~~~~~~~~~~~~~~~
-
     Added
     ^^^^^