[Docs/Workflow] Fix installation bugs/issues for conda/uv/venv (#7466)

# Description

Downloaded Isaac Sim packages linked through `_isaac_sim` were still
allowed to select conda, uv, or venv Python environments even though
that workflow is no longer supported. The CLI warnings and launcher
behavior also contradicted the updated installation documentation.

This PR:

- rejects downloaded Isaac Sim packages combined with conda, uv, or venv
environments in the CLI and platform launchers;
- marks live Isaac Sim source builds so their supported uv workflow
remains available;
- removes the nine orphaned installation include pages and cleans up
stale references;
- documents the breaking workflow change in the 3.0 release notes and
package changelog;
- synchronizes the installation skill guidance; and
- adds regression coverage for downloaded-package and source-build
behavior.

No additional dependencies are required.

## Type of change

- Bug fix
- Breaking change
- Documentation update

## Release backport

- [x] <!-- backport-active-release --> Backport this pull request to the
active release branch after it merges into `develop`

## Screenshots

Not applicable.

## Validation

- `uv run --frozen python -m pytest
--confcutdir=source/isaaclab/test/cli
source/isaaclab/test/cli/test_env_commands.py
source/isaaclab/test/cli/test_install.py
source/isaaclab/test/cli/test_misc_commands.py` — 34 passed
- `uv run --isolated --extra test -- make -C docs current-docs` — passed
without warnings
- `uv run isaaclab -f` — passed
- `uv run --no-project python tools/skills/cli.py check` — passed
- `uv run --frozen python tools/changelog/cli.py check upstream-develop
--include-worktree` — passed

## Checklist

- [x] I have read and understood the contribution guidelines
- [x] I have run the pre-commit checks
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove the fix is effective
- [x] I have added a changelog fragment for the touched package
- [x] My name already exists in `CONTRIBUTORS.md`

Author: StafaH

docs/conf.py

@@ -211,7 +211,6 @@ def _read_pinned_versions() -> dict:
     "licenses/*",
     "plans",
     # Include-only fragments (pulled in via ``.. include::``; not standalone pages).
-    "source/setup/installation/include/*",
     "source/migration/include/*",
 ]