feat: support generic markdown file path reference

[ulivz]

cc @yyx990803.

Summary

Reproduction Link

• https://vue-test-utils.vuejs.org/api/#mount (Click: See also:Wrapper, it will go to homepage.)
• http://www.v2js.com/vuepress-link/ (This link gives all possible situations)

Reproduction

Assuming you have following directory structure:

docs
├── README.md
└── guide
    ├── README.md
    ├── item-1.md
    └── item-2.md

When you use [~](guide/README.md) at docs/README.md, the expected output href should be /guide/ instead of /.

Also, when you use [~](README.md) at docs/guide/item-1.md, the expected output href should be /guide/ instead of /.

With this PR, you can even use ../.

More

This PR also supports the usage of relative paths for all cases and doesn't force a absolute paths beginning with /. so vuepress's intrusion on md files has also been reduced to a minimum. You can safely migrate to vuepress from other static website builders without having to modify the links in the markdown file。

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information: