Document path to regexp migration

[euoia]

Add documentation based on the updates to path-to-regexp in https://github.com/pillarjs/router/pull/102/files.

I also took the opportunity to simplify the intro and remove references to "alpha". I'm happy to revert or extract these to a separate pull request if it's more appropriate.

[euoia]

See the original version of this pull request if you need more context: https://github.com/expressjs/expressjs.com/pull/1316

[euoia]

@dougwilson @crandmck The migration guide is translated into other languages. Can you let me know the process for handling the translated versions?

[dougwilson]

Hi @euoia sorry for the delay. I will take a read through this, but first just wanted to respond to your question:

Translation is now only handled by community members who lend a hand; we no longer have a professional translation service. If the English page changes enough to make the translated page misleading or such, we can just delete it so that lanage sees the English version until someone translates it again.