[docs] Update Fontsource installation instructions

[ayuhito]

Fontsource went through a breaking change to make the library more versatile for international users.

This update aims to clean up all the old documentation to be more reflective of that and easier to understand for the reader. Also, the repository moved to an org so the main link was edited to not waste a redirect.

• I have followed (at least) the PR section of the contributing guide.

[mui-pr-bot]

Details of bundle changes

Generated by 🚫 dangerJS against 0d31e80

[vhakulinen]

Shouldn't this be mentioned in the 4.x docs too? The 4.x mentions this:

⚠️ Be careful when using this approach. Make sure your bundler doesn't eager load all the font variations (100/300/400/500/700/900, italic/regular, SVG/woff). Fontsource can be configured to load specific subsets, weights and styles. Inlining all the font files can significantly increase the size of your bundle. Material-UI default typography configuration only relies on 300, 400, 500, and 700 font weights.

I think that the 5.x docs helps the user much better that 4.x on this topic.

[oliviertassinari]

All ongoing work is on v5. If we cherry-pick this commit to v4, we would want to cherry-pick a lot more for consistency in the strategy. If you won't bother about breaking changes, you can use v5-alpha. We aim for the same level of quality as for a stable release.