add r/webpack page

[chenxsan]

An experiment for https://github.com/webpack/webpack/pull/13822#pullrequestreview-709439528

1. from: https://webpack.js.org/r/webpack/?option=output/filename
2. to: https://webpack.js.org/configuration/output/#outputfilename

Currently we can only make those redirections happen on client side as a single page application hosted on github pages won't support any server side redirects, i.e., urls of https://webpack.js.org/webpack-options/output/chunk-load-timeout type won't work unless we generate every intermediate page for them.

Example: https://webpack-js-org-git-fork-chenxsan-feature-ha-97c6dc-webpack-docs.vercel.app/r/webpack/?option=output/filename

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[chenxsan]

I'm afraid these's no one-size-fits-all solution as the documents don't share a common pattern.

E.g.,

1. /configuration/module/#modulegenerator
2. /configuration/module/#rulelayer

This pull request would work only for the first pattern. And there could be other patterns besides the aforementioned two.

Anyway we can maintain a redirect map like below on this repository:

{
  'output/filename': '/configuration/output/#outputfilename',
}

But I don't think it's a good idea, there could be hundreds of options. Generally I think we should just let users take advantage of the search function on our documentation site.