[Feature]: dynamic base href support

[JounQin]

What problem does this feature solve?

We want to deploy our documents into multiple sites, for siteA its base could be /docs, but for siteB its base could be /platform-docs, for now we have to build them multiple times separately.

Warning

This feature could break SSG

What does the proposed API look like?

Proposal:

1. check typeof __RSPRESS_BASE_HREF__
2. check base#href
3. provide function support for runtime check

[Timeless0911]

I think building multiple times is a reasonable solution, and all other solutions will increase the complexity.