[upgrade guide] [i18nIgnore] Add upgrade to v2 guide + moving pieces

[sarah11918]

Creates an "Upgrade v1 -> v2" guide, and makes all the necessary infrastructure changes:

• adds "Upgrade to v2" in the Start Here section of the nav bar
• creates a new /guides/upgrade-to/v2.mdx page
• renames the previous migrate.mdx file to /guides/upgrade-to/v1.mdx (this can eventually be deleted if we decide. "migrating to older versions?" link will go to a fixed GitHub commit version of this file so that it can be removed from docs)
• redirects /en/migrate/ and older existing /en/migrate/0.21.0 redirect to /en/guides/upgrade-to/v2 in netlify.toml instead of an Astro page file.
• references this guide on the Getting Started docs landing page

Thank you @delucis for suggestions re: finer strategy points!

• [x ] TODO: Write the content! 😅
  (Will start by mostly faithfully reproducing Changelog content; update to better "guide" content incrementally.)

ALSO TODO:

• Convert code samples to Docs (not GitHub) format
• Decide/consitenc-ize how we refer to Astro versions
• ~~[x] reference new guide on main Getting Started page ~~ (Will make separate PR that's not ignored by translators)
• decide for sure if this belongs in root, or under "guides"
• if under guides, then update netlify.toml redirects
• decide whether to link to old v1 upgrade guide on GitHub or in docs (currently GitHub)

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[delucis]

Left a note about redirects. Will you delete the migrate-to-v1.mdx file before merging? Don’t think we need to keep that content around.

I think you can delete the translations of /migrate/ in this PR too. That would fix the “Mismatched Slugs” check.

[yanthomasdev]

withastro/astro PR fixing the last broken links: withastro/astro#5808 (marked as a draft but can be merged at anytime)

[sarah11918]

Thanks for the excellent review @delucis! I'm happy to go through and reorder for impact.

You make a great point, probably completely removing deprecated things probably affects the fewest projects , even if it they in a way are the most "final" absolutely breaking changes.

I'll take a quick pass at "importance/impact" now, to at least get start with importance because we can probably improve it immediately. Would love to dig into the data to verify some assumptions, if that's something we can get around to!

[yanthomasdev]

Great work @sarah11918, I added a few suggestions but LGTM!

[delucis]

Quick couple of notes but this looks good! I also still need to add a note for the changed behaviour in the latest Svelte integration.