docs(website): add the v7 to v8 migration guide

[unional]

Adds guides/migrating-to-v8: every change between type-plus@7.6.2 and 8.0 that breaks a build or changes a result. Each entry gives the change, the reason, and a before and after example. A checklist sits at the top.

How the list was built

• Every major and minor changeset in .changeset/, and the 8.0.0-beta.* entries in CHANGELOG.md.
• A diff of the export surface: the published 7.6.2 esm/index.d.ts against the current build, via the compiler API. This found about 100 removed names, including the whole XxxType / NotXxxType / IsStrictXxx family and Positive / Negative / Integer / IsWhole. No pending changeset covers those.
• The v7 "before" results were read off the 7.6.2 declarations, not the changesets. Some changesets describe beta states, not 7.x. For example, CanAssign<never, number> was false in 7.x, not never, and v7 IsPositive had no exact option.
• Every v8 "after" example was pinned in a throwaway spec with testType.equal, which compiled clean on TS 6.0. The spec was deleted before this commit.

Findings worth a look

• v8 predicates distribute over unions by default. IsString<'a' | 1> was false in 7.x and is boolean in v8. IsTrue<boolean> went from false to boolean. No changeset mentions this, and it is the change most likely to catch users silently. The page puts it first under "Changed results", with { distributive: false } as the fix.

Other changes

• Sidebar entry under Guides.
• A link from readme.md and packages/type-plus/readme.md, in the v8 beta status section.
• migrating-then-else-to-options.mdx: removed the claim that NotUnknownOr keeps a positional Else. It takes { $unknown } since refactor(type-plus)!: one options convention for transforms #721.
• src/object/readme.md: ObjectPlus.Merge no longer shows the removed Options parameter. The legacy tree already records the other removals.
• Patch changeset, because both readmes ship in the tarball.

pnpm verify passes locally, including the website build.

🤖 Generated with Claude Code

[changeset-bot]

🦋 Changeset detected

Latest commit: 214a71c

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
type-plus	Patch

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (eb8d2cb) to head (214a71c).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #797   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files           40        40           
  Lines          173       173           
  Branches        30        30           
=========================================
  Hits           173       173           

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