docs(site): show GitHub stars and PyPI downloads badges

[ekalinin]

What changes

The landing page hero gets two more badges after the existing three:

• GitHub stars, linking to the repository. The stargazers page is not linked because it returns 404 without a login.
• PyPI downloads per month, linking to the nodeenv page on pypistats.org.

Both are shields.io badges, so the counts update themselves. The page still has no JavaScript, no build step and no counts written into its text. The stars badge uses style=flat so it matches the other badges instead of shields' social style.

Most of the monthly downloads come from installs of pre-commit, which depends on nodeenv. The badge says "downloads", which is what it counts.

Checked

• At 1280 px all five badges fit on one row. At 375 px the row wraps onto three lines and the page does not overflow.
• With images blocked, both new badges fall back to their alt text.
• A network log of the page shows only the five badge requests.
• Both badge links return 200.