[WIP] Feature/vuepress docs

[NataliaTepluhina]

Short description and why it's useful

All Vue-Storefront docs reviewed and moved to Vuepress platform. To check the current view of docs, run vuepress dev docs or npm run docs:dev

Contribution and currently important rules acceptance

• I read and followed contribution rules
• I read the TypeScript Action Plan and adjusted my PR according to it
• I read about Vue Storefront Modules and refactoring plan for them

[pkarw]

This is really cool!

[filrak]

One thing - can this vuepress instance be a separate package in monorepo so we don't need to have vuepress dependency in our main package.json?

[pkarw]

@NataliaTepluhina can You please add this new config option to the docs - #1764 ?

[NataliaTepluhina]

@NataliaTepluhina can You please add this new config option to the docs - #1764 ?

Added to docs.
@pkarw could you please also provide a short description for "multisiteCommonCart"?

[pkarw]

I'm merging this in for 1.4 - please create new PR with further works