[doc] Make the link to documentation shorter and memorizable?

[archibate]

Shall we move:
https://taichi.graphics/versioned_docs/develop/documentation/advanced/meta.html
to:
https://taichi.graphics/docs/develop/advanced/meta.html

[rexwangcc]

I think that's a good idea. Actually I was hisitated about using versioned_docs initially since it's too long. The only concern is that if:

- https://taichi.graphics/versioned_docs/develop/documentation/advanced/meta.html
+ https://taichi.graphics/docs/develop/advanced/meta.html

will make the doc writers a bit more confusing about where to go to write docs, since the URL is bond to the file structure, which means all stuff under website/docs/versioned_docs will be moved to website/docs/docs.

[archibate]

So why not just move /website/docs/src to /src?

[rexwangcc]

So why not just move /website/docs/src to /src?

Sounds good, we could look into it, especially before more docs come in.

We didn't go with a more flat structure simply because the vuepress scaffold made some decisions around the layout, see "VuePress follows the principle of "Convention is better than configuration". "

[rexwangcc]

Resolved by #15