Specify description metadata for each docs page

[billy1624]

Summary

Now the description under each Google search result is basically the first line of sentence on the page. Which is not SEO friendly. We should follow the guide on https://docusaurus.io/docs/seo#single-page-metadata to specify a meta description for each of it.

https://www.google.com/search?q=site:sea-ql.org+SeaORM