docs: changing title for 1.x and update readme

[ulivz]

It's still some time before 1.x is officially released. To avoid users misreading the document and clicking on that annoying dialog box every time they enter the website, the following changes are made:

• Temporarily change the docs’ urls as follows:
  • 0.x docs:
    • Before: https://v0.vuepress.vuejs.org
    • After: https://vuepress.vuejs.org
  • 1.x docs:
    • Before: https://vuepress.vuejs.org
    • After: https://v1.vuepress.vuejs.org/
• Temporarily change the 1.x docs's title to VuePress 1.x
• Delete the annoying dialog box

// @cc @yyx990803