[Docs] Rework ecosystem page

[StafaH]

Description

Reworks the Isaac Lab ecosystem page to:

• explain Isaac Lab's role as a robot-learning framework rather than a simulator
• show how tasks, backends, and learning workflows fit together
• compare the supported physics and rendering runtimes
• clarify manager-based and direct environment authoring
• provide a concise guide to the repository packages and their roles
• summarize the relationship to earlier Isaac projects

The new ecosystem animation is intentionally not included yet. The page references source/_static/setup/ecosystem.gif; that asset will be added in a follow-up commit from a media-capable machine before this PR is marked ready for review.

Type of change

• Documentation update

Release backport

• Backport this pull request to the active release branch after it merges into develop

Screenshots

Pending the ecosystem GIF follow-up.

Validation

• git diff --check
• uv run isaaclab -f (all applicable formatting and RST checks pass; the local changelog hook misidentifies unrelated fork changes because this clone's origin points to the fork rather than upstream)
• uv run --isolated --extra dev -- make -C docs current-docs (expected warning: source/_static/setup/ecosystem.gif is not present yet)

Checklist

• I have read and understood the contribution guidelines
• I have run the complete pre-commit checks
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works (not applicable to this documentation-only change)
• I have added a changelog fragment for every touched package (no source package is touched)
• My name already exists in CONTRIBUTORS.md