feat(ci): add redirect checks and reconstruct redirects array for all links from the previous TON Docs

[novusnota]

Closes #123 by adding four kinds of redirect checks:

• all sources must be unique
• all destinations must exist (if they are local)
• changes against upstream docs.json structure links must be accounted for
• changes against previous TON Docs links must be accounted for

Out of them, two are expected to fail — we don't have redirect files for all declared routes AND we don't cover all links from the previous TON Docs. Yet.

All checks pass, see this message for details.

[github-actions]

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[novusnota]

Let's add all missing redirects and/or add TODO stubs for destinations and resolve those in #39.

[novusnota]

Let's add all missing redirects and/or add TODO stubs for destinations and resolve those in #39.

Done!

Instead of explicit "TODO" redirects, which would've led to 404 pages, I've opened new issues and linked to them. And also linked to existing issues when there's no corresponding page stub.

Besides, the "upstream docs.json" check is commented out — it shall be enabled only right before the release. There's a note for that in #39.

[github-actions]

No documentation issues detected.

[github-actions]

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