Allow marking a documentation page as the landing page

[jcomack]

🚀 Feature

Allow maintainers to configure a documentation page as their landing page.

Have you read the Contributing Guidelines on issues?

Yes

Motivation

Currently, Docusaurus allows users to setup in a 'docs only' mode, which requires setting your routeBasePath to an empty string and redirecting visitors by altering the /src/pages/index.js file so it points to your desired destination.

However, this means you're (needlessly) redirecting a visitor to a different URL and thus are leaving the original 'landing page' that you'd expect to see when visiting https://myawesomedocs.com/, for example. Manually recreating the layout of your documentation pages in the index.js file is also an option, but seems cumbersome.

Pitch

Allowing maintainers to mark a particular documentation page as their landing page, would get rid of having to change the settings/files I mentioned before and make it easier for people to get a 'docs only' site up and running. Additionally, I believe it could also benefit in terms of SEO, as you aren't redirecting people "for no reason".

[yangshun]

We could add an option to the @docusaurus/plugin-content-docs to specify the root route to map to another doc route.