fix file paths and hyperlink syntax in README.md

[cocoliliace]

• src/app.rs doesn't exist. Likely a copy-paste error from start-axum
• change frontend/lib.rs to frontend/src/lib.rs to be more accurate
• (https://playwright.dev)[Playwright]: (https://playwright.dev)[Playwright]
  [Playwright](https://playwright.dev): Playwright

By the way, I think the project description on github should say 0.8 instead of 0.7 now?