fix(v2): treat mailto and tel links properly

[lex111]

Motivation

Resolve #2568

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Use the similar elements as the item link in navbar and footer.

        {
          label: 'Mailto link',
          href: 'mailto:foo@bar.com',
          position: 'right',
        },

            {
              label: 'Telephone link',
              href: 'tel:+1234567890',
              position: 'right',
            },

Related PRs

(If this PR adds or changes functionality, please take some time to update the docs at https://github.com/facebook/docusaurus, and link to your PR here.)

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