docs: add B20 concept docs

[soheimam]

What changed? Why?

Added b20 concepts

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[cb-heimdall]

🟡 Heimdall Review Status

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[roethke]

Two issues, both from SEIZE_RECEIVER_POLICY (the 6th policy scope) being omitted. Inline with comments left in #1766.

Separately, on docs.json: there is no redirect for the exact old path /base-chain/specs/upgrades/beryl/b20 (the deleted single-page spec lived there). Redirects were added for /base-chain/b20, /base-chain/b20/*, and …/beryl/b20/{concepts,implementation,reference}/*, but not the bare …/beryl/b20, so that URL — and old deep links to its anchors (#multiplier, #policy-registry, #pause, #announcements), will 404 after merge.

[roethke]

SEIZE_RECEIVER_POLICY fixes look good, thanks. One issue with the new redirect, inline.

[roethke]

Checked preview and this looks good, just need to resolve the conflicts then we can approve/merge