Feature request: Allow dynamic github edit links

[gskril]

It would be helpful if editLink.pattern accepted a function with a path argument so the "Edit on Github" link could be customized per page.

Ideal usage would look something like this:

editLink: {
    pattern: (path) => {
      if (path.startsWith('/ensip/')) {
        return `https://github.com/ensdomains/ensips/edit/master/ensips/${path}.md`
      }

      return `https://github.com/ensdomains/docs/edit/master/src/pages/${path}`
    },
    text: 'Edit on Github',
  },

An example use case is ENS, where our ENSIP pages are generated from another repo at build time.

Related to ensdomains/docs#446