Move component v-model to its own page

[skirtles-code]

Motivation

The docs for component v-model are currently located at the end of the page about component events. I think that made sense historically, but there's now enough material on both topics to justify splitting them into two pages.

While it's clearly necessary to document component events before component v-model, I don't think they're really the same topic. I frequently find myself linking to the docs for component v-model, and it took me a while to 'learn' that they lived at the bottom of the page about events. Time and again I would look for a separate page, like the page about 'Form Input Bindings' for using v-model on native elements.

Redirects

I've updated internal links, but I've also tried to provide redirects for external links to the v-model docs. See the code at the top of events.md for details.

Notes for translators

I've tried to make minimal changes to the text itself. The new v-model.md should contain almost the same content that was in events.md. The only intentional changes I've made to the text are:

• Updating the headings. In particular, headings are now ## instead of ###.
• The top of the page, up to the first example. The previous wording assumed it was part of the page about events, so I've written a brief new introduction.

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[yyx990803]

I like this idea.

[NataliaTepluhina]

@skirtles-code great work on moving v-model to a separate doc!