docs: Batch 7 — governance deep dives (#216)

## Summary

- **Expanded**: \`docs/concepts/governance.md\` — added 5 new sections:
neuron hotkeys, following rules (thresholds, catch-all, periodic
confirmation), voting thresholds (absolute majority / 3% quorum),
maturity operations (disburse/stake/auto-stake + modulation formula),
voting rewards formula R(t), and the Neurons' Fund (with disabled status
note)
- **New reference**: \`docs/references/nns-proposal-types.md\` — all 11
NNS proposal topics and their proposal types with descriptions, derived
from the full Learn Hub staging article
- **New concept page**: \`docs/concepts/sns-framework.md\` — SNS
architecture (SNS-W, upgrade paths, SNS subnet), launch process (swap
mechanics, min/max thresholds, fallback), SNS neurons (public, flexible
permissions, configurable voting power), SNS proposals (critical vs
non-critical topics, built-in + custom proposals), configurable voting
rewards; includes SNS rewards graph image
- **New reference**: \`docs/references/sns-settings.md\` — full
\`NervousSystemParameters\` reference with types and descriptions for
all fields (renamed from \`sns-dao-settings.md\` to match the "SNS
Settings" title)
- **Updated**: \`docs/guides/governance/managing.md\` — replaced 2 Learn
Hub links with internal paths (\`references/sns-settings.md\` and
\`concepts/sns-framework.md#sns-neurons\`)
- **Updated**: \`sidebar.mjs\` — added \`concepts/sns-framework\`,
\`references/nns-proposal-types\`, \`references/sns-settings\`
- **Deleted**: 14 consumed Learn Hub staging files (7 from \`nns/\`, 7
from \`sns/\`)

## Staging files deleted

-
\`.migration/learn-hub/how-does-icp-work/nns/{overview,neurons,proposals,neuron-attributes,neurons-fund-nf,voting-rewards,proposal-topics-and-types}.md\`
-
\`.migration/learn-hub/how-does-icp-work/sns/{sns-service-nervous-system,framework-and-architecture,launch,neurons,proposals,rewards,dao-settings}.md\`

## Sync recommendation

\`governance.md\` additions: \`informed by Learn Hub articles
"Overview", "Neurons", "Proposals", "Neuron Attributes", "Neurons' Fund
(NF)", "Voting Rewards" (migrated, source retired)\`
\`nns-proposal-types.md\`: \`informed by Learn Hub article "Proposal
Topics and Types" (migrated, source retired)\`
\`sns-framework.md\`: \`informed by Learn Hub articles "SNS — Service
Nervous System", "Framework and Architecture", "Launch", "Neurons",
"Proposals", "Rewards" (migrated, source retired)\`
\`sns-settings.md\`: \`informed by Learn Hub article "DAO Settings"
(migrated, source retired)\`

Author: marc0olo