docs: correct the changelog guidelines

[upbqdn]

Motivation

Closes #11123.

Solution

The merge-policy half of the issue landed in #11285, so this is the changelog
half only, rebased onto current main, where the changelogs have since moved to
changie. One commit on book/src/dev/changelog-guidelines.md:

• Section order. The list cited Keep a Changelog order while leading with
  Breaking Changes. That section is real — it is the breaking kind in
  .changie.yaml — so the text now says the order is the kinds list: Zebra's
  breaking first, then Keep a Changelog order.
• Double-listing exception dropped. It contradicted the one-entry-per-change
  rule stated just above it. The priority order already files a
  breaking-and-additive change under Breaking Changes; the entry just has to
  name the new capability too. The text now draws the distinction the exception
  was blurring: zebra-chain 12.0.0 lists fix(chain)!: validate the total amount when updating a value balance #10817's new
  ValueBalanceError::Total under Breaking Changes and its new
  ValueBalance::total under Added because those are two changes, not one
  change filed twice.
• New Dependency updates section. A crate changelog needs an entry when a
  dependency moves to a semver-incompatible version whose types appear in the
  crate's public API: the two versions do not unify, so consumers have to move
  with the bump. That is breaking, so the fragment kind is breaking —
  .changie.yaml maps Changed to a minor bump, which would ship a consumer
  build break in a minor release. A semver-compatible bump unifies, so it gets a
  Changed entry only when consumers notice it. Nothing covered any of this,
  and read out of context the zebrad exclusion looks global — which is how the
  zcash_primitives 0.30.0 entry came to be dropped in chore: Update to zcash_primitives version 0.30.0 #11111. The zebrad row
  now says it applies to that file only, and the "deciding where to document"
  table gains the dependency and MSRV rows.
• Stale versions. The compatibility template quoted NU6.1 and Rust 1.75; it
  now shows NU6.3 and the 1.91 zebrad / 1.88 workspace split, and says to fill
  the table from rust-version rather than copy it. The crate-breaking table's
  MSRV row quoted 1.70 to 1.75; it now reads "rust-version raised to a newer
  stable", which cannot go stale.

Tests

Docs-only: no code path changes, so no test applies. What the text now asserts
is checkable against the repo instead — the kind list and its bump levels
against .changie.yaml, the two-changes example against
.changes/zebra-chain/v12.0.0.md, the dependency example against
zebra_rpc::MinerParams::addr and the zcash_keys version in the workspace
manifest, and the MSRV values against rust-version.

AI Disclosure

• No AI tools were used in this PR
• AI tools were used: Claude, to find the inconsistencies and draft the changes.

PR Checklist

• The PR title follows conventional commits format: type(scope): description
• The PR follows the contribution guidelines.
• This change was discussed in an issue or with the team beforehand.
• The solution is tested.
• The documentation and changelogs are up to date.

[v12-auditor]

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