Add Dart Map .addEntries() term entry

[AI-Explorer25]

Description

Added a new term entry for the Dart Map method .addEntries(). This entry is located at:

docs/content/dart/concepts/map/terms/add-entries/add-entries.md

The entry includes:

• Metadata with appropriate tags (Dart, Map, Methods) and catalog content.
• A detailed description explaining how .addEntries() allows adding multiple key-value pairs to a Map using an iterable of MapEntry objects.
• A Syntax section showing the proper usage of .addEntries().
• An Example section demonstrating multiple use cases, including creating an iterable of MapEntry objects and adding them to an existing Map.
• Codebyte examples showing practical usage with expected output.

This entry follows the existing patterns and structure used in other Dart term entries, such as .length and .keys.

Issue Solved

Closes #8085

Type of Change

• Adding a new entry
• Editing an existing entry (not applicable here)
• Updating the documentation (not applicable here)

Checklist

• [x] All writings are my own.
• [x] My entry follows the Codecademy Docs style guide.
• [x] My changes generate no new warnings.
• [x] I have performed a self-review of my own writing and code.
• [x] I have checked my entry and corrected any misspellings.
• [x] I have made corresponding changes to the documentation if needed.
• [x] I have confirmed my changes are not being pushed from my forked main branch.
• [x] I have confirmed that I'm pushing from a new branch named after the changes I'm making.
• [x] I have linked any issues that are relevant to this PR in the Issues Solved section.

[CLAassistant]

All committers have signed the CLA.