Fix stale hyperlinks in README

[rayfix]

Fix broken hyperlinks in README

Motivation:

Fix broken hyperlinks in the README.

Modifications:

Changed the links pointing to the documentation of various protocols and classes. For Bootstrap-related links, centralize the definitions of the links so that they are easy to change if they break in the future. This follows the convention of the current document.

Result:

Links that resolve. 🥇

[swift-server-bot]

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[swift-server-bot]

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[swift-server-bot]

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[swift-server-bot]

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[swift-server-bot]

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[swift-server-bot]

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[swift-server-bot]

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[swift-server-bot]

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[swift-server-bot]

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[swift-server-bot]

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[swift-server-bot]

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[rayfix]

Wow, the swift-server-bot seems really eager to get a review. Haha.

[Lukasa]

Thanks for this patch!

The links aren't stale, they're actually too fresh. @tomerd, it seems like our docs haven't built in a while. Any idea what's going on?

[Lukasa]

Ping @tomerd

[Lukasa]

Ping @tomerd

[tomerd]

sorry about the delay @Lukasa, looks like SourceKitten is segfauling when trying to generate the docs. Will look into it

[tomerd]

https://github.com/apple/swift-nio/pull/2013