Dynamic routes revision

[Jutanium]

What kind of changes does this PR include?

• New or updated content

Description

• Closes #1631
• Introduces the different setups and behavior for dynamic routes in SSR vs SSG
• Introduces Astro.params alongside getStaticPaths, rather than in a separate section. This matches the tutorial, but also avoids having to repeat examples to show the full picture.
• Removes incomplete "examples" that don't show the implementation
• Reimplements the final example so that it works in SSR

Right now, much of the "guide"-style content for getStaticPaths is in the references section. A future PR would restructure and bring that information here.

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[Jutanium]

@delucis and @kevinzunigacuellar both liked the GitHub file routes example, and even though it isn't "a real example", it illustrates the combination of rest parameters with other named parameters really well. Added back!

[sarah11918]

LGTM!