docs: retire pre-Nexus content, fix broken links, and pin webpack

[guilhermefleury]

Summary

Brings the docs back in line with Syscoin Core 5.x (Nexus). Mix of bug
fixes and content modernization, kept as separate commits for granular
review:

1. fix: replace broken external links across docs — link-rot sweep.
   Replaces 10 dead external URLs with current equivalents (faucet.rollux.com,
   blockbook-dev.syscoin.org, etc.) or drops links where there is no
   working replacement.

2. fix: pin webpack to 5.75.0 — adds resolutions / overrides to
   package.json. Newer webpack 5.10x rejects the options webpackbar 5.0.2
   passes to its ProgressPlugin. Pin restores a working dev server.

3. docs: retire pre-Nexus SPT content (Syscoin Core 5.0) — the 4.x
   asset RPC family (assetnew, assetupdate, assetallocationsend, etc.)
   was removed in Core 5.0; see release notes §9. Removes the four
   docs/guides/spts/*.mdx how-to guides and asset-index.mdx (all teach
   removed RPCs), rewrites the surviving SPT-adjacent pages
   (tech/tokens, tech/notary, dev-resources/sys/exchange-integration,
   dev-resources/sys/z-dag) for current 5.x reality.

4. fix: rename args on sentrynode_setup.mdx — protx_register_prepare
   args renamed in current Core (ownerKeyAddr → ownerAddress,
   votingKeyAddr → votingAddress, feeSourceAddress → fundAddress),
   plus removal of the signmessagebech32 RPC (replaced by signmessage
   which now handles bech32), plus the 4.3.0 version references updated to
   5.x.

Verification

• Every removed RPC verified absent from current Syscoin Core master via
  grep over src/
• Every replaced URL verified live via HTTP probe
• Every renamed arg verified against the current RPCHelpMan block

Per-commit review

The 4 commits are independent — reviewers can read each commit's diff
on its own. Earlier sections of any one commit do not depend on later ones.