Update installation instructions to the uv workflow

[marcelolafleur]

This brings the installation docs in line with what actually works, following the pattern of EAPD-DRB/OG-PHL#79.

What a user runs into today:

• The README's first recommendation is pip install ogbra, but the ogbra package has never been published to PyPI, so that command fails for everyone (verified: "No matching distribution found for ogbra", tried on Python 3.9.6 and confirmed against the PyPI index). The PyPI badges in the header point at the same nonexistent package.
• The contributor guide still walks new contributors through creating a conda environment, but environment.yml was deleted in the 0.1.0 uv migration (Migrate from conda to uv #5), so conda env create fails outright. It also tells people to run a test suite with OG-USA's markers (needs_puf, regression, "close to 24 hours") that don't exist here, and to sync with a master branch this repo doesn't have.

What this PR does:

• README: replaces the PyPI section with an Installation section that has two levels — the OG family universal installer (--repo og-bra) as the easy way, and a manual uv install — written so a novice can copy-paste each block, with separate macOS, Linux, and Windows instructions, prerequisites included, and a note on what to expect from the example run (runtime, output locations, and that the UN data token prompt can be answered by just pressing return).
• Contributor guide: setup now installs uv and runs uv sync --extra dev; the test command is the repo's real one (uv run python -m pytest -m "not local"); master references are now main.
• CHANGELOG entry.

What was tested: from a fresh clone of this repo, the manual path (uv sync --extra dev, import, fast test files — 11 passed) works end to end on macOS; the universal-installer path was run end to end against this repo and completes in about half a minute. Lint is green. No changes to uv.lock or any code.

One dependency: the --repo og-bra shortcut in the README needs the installer catalog entry in PSLmodels/OG-Core#1179 to merge first. Until then the installer path works via --repo-url https://github.com/PSLmodels/OG-BRA.git.