MNT: Automate CHANGELOG.md updates via GitHub Actions #905

Is your feature request related to a problem? Please describe.
Currently, the CHANGELOG.md file is manually updated by contributors in every Pull Request. This process has several downsides:

1. Human Error: Contributors often use the wrong format, forget to add links, or place entries in the wrong section.
2. Merge Conflicts: Since almost every PR touches the [Unreleased] section of the changelog, it is a frequent source of merge conflicts that require manual resolution.
3. Inconsistency: The descriptions often vary in style (e.g., starting with "ENH", "BUG", or just plain text).

Describe the solution you'd like
I would like to implement a GitHub Action (or a bot) that automatically updates the CHANGELOG.md file when a PR is merged.

The automation should:

1. Read PR Metadata: Extract the PR title, number, and URL.
2. Categorize: Use GitHub Labels (e.g., Enhancement, Bug, Refactor, Documentation) to decide which section of the Changelog the entry belongs to.
3. Format: Automatically prepend the standard prefixes (ENH, BUG, DOC) and append the PR link [#123](url).
4. Write: Append the new entry to the [Unreleased] section of CHANGELOG.md.

Proposed Logic for Categorization
The automation should map GitHub Labels to "Keep a Changelog" sections:

• Label: Enhancement or Feature $\rightarrow$ Section: ### Added
• Label: Bug $\rightarrow$ Section: ### Fixed
• Label: Refactor or Maintenance $\rightarrow$ Section: ### Changed
• Label: Documentation $\rightarrow$ Section: ### Added or ### Changed (depending on configuration)

Potential Tools
We should investigate tools such as:

• Release Drafter: Creates a draft release with the changes, which can then be pasted into the Changelog.
• Git Cliff: A highly customizable changelog generator.
• Towncrier: (Used by pandas/scipy) Uses fragment text files to generate the changelog at release time to avoid merge conflicts completely.

Additional context
The current format of CHANGELOG.md adheres to Keep a Changelog. The automation must respect this format.

Example of a desired entry generation:

• PR Title: "Add new drag model"
• PR Number: 895
• Label: Enhancement
• Result: - ENH: Add new drag model [#895](https://github.com/RocketPy-Team/RocketPy/pull/895) under ### Added.