feat(seo): add link tag to frontmatter allowed values

[Slashgear]

Summary

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:

• link is a reserved frontmatter key.

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

For SEO purpose, a Vuepress user should be able to add link tag in head.
For example, user could need to define canonical url.

• Related documents have been updated
• Related tests 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:
Should we add a update-link mixin too ?

[ulivz]

Such changes does not work under dev.

[Slashgear]

@ulivz That what i mean with my question about creating a mixin like the existing update-meta one. Should i do it ?

[ulivz]

We're closing this pull request as stale as it's more than 20 days without activity, and without an associated Pull Request. Please feel free to continue discussion. We'll reopen this issue if anything actionable is posted.