Add sb3-plus to projects page (resolves #527)

[Copilot]

This PR adds @adysonmaia's sb3-plus project to the projects page, addressing issue #527 which requested multi-output policy support for Stable-Baselines3.

Background

Issue #527 was opened requesting support for multi-output policies and dictionary action spaces in SB3. This feature is particularly useful for complex environments that require multiple types of actions (e.g., discrete and continuous actions simultaneously). While this feature wasn't implemented directly in the main SB3 library, @adysonmaia developed an extension called sb3-plus that provides PPO with dict action space support, enabling independent action spaces.

Changes

• Added sb3-plus project entry to docs/misc/projects.rst positioned after the sb3-extra-buffers section (as both are SB3 extensions)
• Updated changelog in docs/misc/changelog.rst to document the addition under the current WIP release documentation section

The project entry describes how sb3-plus extends SB3 with multi-output policy capabilities and provides a practical solution for environments requiring complex action scenarios, directly addressing the community need expressed in #527.

This addition helps users discover the sb3-plus extension and provides an official acknowledgment of this valuable community contribution that solves a long-standing feature request.

Original prompt

To solve #527, please add @adysonmaia/sb3-plus to the project page (similar to what is done for SB3 extra buffers), don't forget to update the changelog

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