Update README to clarify project site configuration requirements with tools like CRA, webpack, Vite, etc.

[Nezteb]

I've noticed a common misconception about GitHub Pages, specifically project sites that are not served at the root <user|org>.github.io domain and instead under a /<repo> path.

Some example issues for the gh-pages project that reflect this:

• Build files not found net::ERR_ABORTED 404 #285
• Set homepage through gh-pages cli #233
• main.js 404 not found and blank page when I deploy (webpack-react manually configuration) #414
• 404 when I deploy  #351
• Messed up CSS after deploying with gh-pages #296
• Configure 404 error page #227
• 404 error  #184

The only way I can think to alleviate this is to address it right in the project's README.md, hence this PR.

[tschaub]

Thanks, @Nezteb