Add changelog guidelines block

[pinkevmladchy]

Add change log guidelines block to each item type at the end of the page.

[Copilot]

Pull request overview

Adds a reusable “Writing the changelog” guidelines section to the end of each ThingsBoard IoT Hub contribution guide page, ensuring consistent instructions across all item types.

Changes:

• Introduced a new MDX include block containing changelog writing guidelines and an example.
• Imported and rendered the new <ChangelogGuidelines /> block at the end of each IoT Hub guide page (device, widget, rule chain, alarm rule, calculated field, solution template).

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/content/docs/docs/iot-hub/guides/widget.mdx	Imports and renders the shared changelog guidelines block at the end of the widget guide.
src/content/docs/docs/iot-hub/guides/solution-template.mdx	Imports and renders the shared changelog guidelines block at the end of the solution template guide.
src/content/docs/docs/iot-hub/guides/rule-chain.mdx	Imports and renders the shared changelog guidelines block at the end of the rule chain guide.
src/content/docs/docs/iot-hub/guides/device.mdx	Imports and renders the shared changelog guidelines block at the end of the device guide.
src/content/docs/docs/iot-hub/guides/calculated-field.mdx	Imports and renders the shared changelog guidelines block at the end of the calculated field guide.
src/content/docs/docs/iot-hub/guides/alarm-rule.mdx	Imports and renders the shared changelog guidelines block at the end of the alarm rule guide.
src/content/_includes/docs/iot-hub/guides/changelog-guidelines.mdx	New shared documentation block describing changelog structure, semver guidance, and a sample entry.

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