docs: make README setup commands cross-platform

[William-Mou]

What this changes

Why

How it was checked

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• A label is set, so this lands in the right release-notes section
  (breaking-change, enhancement, bug, documentation)
• Anything a competitor would notice is in CHANGELOG.md under Unreleased

[Copilot]

Pull request overview

Updates the project README to make setup and usage instructions more cross-platform, especially for Windows shells, by clarifying path conventions and standardizing on uv run examples.

Changes:

• Adds a “Shell and path conventions” section to clarify path separators and repo-root assumptions.
• Updates uv instructions to use uv sync --locked and encourages uv run to avoid shell-specific activation.
• Expands pip instructions with explicit activation steps for PowerShell/cmd/Linux and updates example commands to be more portable.

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[thc1006]

Sorry for the slow reply, this sat for two days before I noticed the request.

TLDR: I ran everything it tells a competitor to run and all of it works, including the pip path CI never touches. Nothing blocking from me. Two notes below, and one thing about develop that is not visible from here.

What I ran

On a clean checkout of the branch, in order:

uv sync --locked                                  installs
uv run python -m unittest discover tests          OK, 22 skipped
uv run python doc/examples/test_navigation_agent.py   runs to completion

and every path the new text points at resolves: BalloonPoppingGymEnv/evaluation/evaluate.py, the config beside it, and the results directory.

Then Option B the way the README now words it, in a fresh venv:

python -m venv .venv
python -m pip install -r requirements.txt
python -c "import BalloonPoppingGymEnv, rocketpy"    both import

That last one is worth saying out loud because CI has a comment admitting it does not cover it: the gating job installs requirements-dev.txt, and requirements.txt is the file a competitor is sent to. So the part of the README this PR expands most is also the part nothing else checks. It works today, on Linux at least.

The one thing I cannot check here

The PowerShell and cmd branches. This machine has neither, so I read them rather than ran them, and I would not claim more than that.

Copilot's point about / and \ looks fair to me, though smaller than it reads: the new section says paths do not need rewriting with \, and then the activation commands are .\.venv\Scripts\Activate.ps1. Those can both be true, since the sentence is about repository paths and the activation script is a Windows path, but a reader hitting them ten lines apart has to work that out. One clause in the conventions sentence would settle it.

Something not visible from this PR

develop is four PRs ahead of main and one of them, #137, edits the same README block your step 2 sits in: it replaces the working example team_secret with a placeholder. I merged your branch into develop locally and git resolves it cleanly, with the result reading correctly, so this is not a conflict to fix.

Worth knowing anyway, because @zuorenchen has since said the example secret was there on purpose so people can test the submit path without registering, and #137 may be reverted. Either way this PR is unaffected.

Small

uv sync to uv sync --locked is the right change and matches what CI does.

Nothing here needs to change for me. Happy either way on the / and \ clause.