Render initial state in evaluate_policy (Fixes #1692)

[midhunxavier]

Description

evaluate_policy() (used by EvalCallback) only called env.render() after
env.step(), so the initial post-reset state of each episode was never rendered.
This adds a single env.render() after env.reset(), before the first
predict(), so the initial state is shown. The final (post-terminal) frame is
intentionally still not rendered because VecEnv auto-resets sub-environments;
this caveat is now documented in the render docstring with a workaround.

This implements the minimal approach @araffin proposed in the issue thread (render
before predict + document the final-frame limitation), not the rejected
VecEnv-subclass approach.

Motivation and Context

closes #1692

• I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)

Checklist

• I've read the CONTRIBUTION guide (required)
• I have updated the changelog accordingly (docs/misc/changelog.md) (required).
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.
• I have opened an associated PR on the SB3-Contrib repository (if necessary)
• I have opened an associated PR on the RL-Zoo3 repository (if necessary)
• I have reformatted the code using make format (required)
• I have checked the codestyle using make check-codestyle and make lint (required)
• I have ensured make pytest and make type both pass. (required)
• I have checked that the documentation builds using make doc (required)

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AI-assistance disclosure (per CONTRIBUTING.md): this change was developed with
the assistance of an LLM (Claude). The approach was proposed by the maintainer
(@araffin) in #1692; I have reviewed and own the change.