Improve OpenSaas's SEO baseline

[sodic]

It's all about that sweet green lighthouse score.

This might come in handy: https://gist.github.com/NeroxTGC/27a588ad5d78cf09a0b4464321eb5921

[NeroxTGC]

This is from a while ago, I don't know if the changes in the template are outdated. I also updated the SEO doc section: #297