ci: add missing path filters so changed inputs trigger their jobs

[DamianReeves]

Problem

Detect Changes gates the Rust and Docs jobs on path filters, but several inputs those jobs actually depend on weren't listed. Changing them ran no jobs, so CI reported green without building anything.

Three cases hit this in the last day:

PR	Change	Rust jobs
#675	rustc 1.97.1 → 1.98.0	skipped
#684	mise tool backend	skipped
#677	morphir-rust submodule bump	ran, but only because it also touched Cargo.lock

A compiler upgrade landed with nothing compiled. That's the same class of blind spot that let the pager clippy lints sit on main unnoticed until #679 — new clippy lints ship with new toolchains.

Changes

rust filter

- 'ecosystem/morphir-rust'   # path-dep source
- '.gitmodules'
- '.config/mise/**'          # pins the Rust toolchain

• ecosystem/morphir-rust — crates/morphir and crates/morphir-live path-depend on crates inside this submodule, so a ref bump changes the source being compiled. A submodule-only bump previously ran nothing.
• .gitmodules — same reasoning, for submodule wiring.
• .config/mise/** — pins the toolchain the jobs build with.

docs filter

- '.config/mise/tasks/ci/validate_docs.py'

The Docs job runs this script, but editing it triggered no job.

Verification

Validated both levels — the workflow itself, and the filters block as dorny/paths-filter parses it (it's a block scalar, so the # lines are YAML comments inside a string):

{
 "rust": ["Cargo.toml","Cargo.lock","crates/**","ecosystem/morphir-rust",
          ".gitmodules",".config/mise/**",".github/workflows/ci.yml"],
 "docs": ["website/**","docs/**","**/*.md",
          ".config/mise/tasks/ci/validate_docs.py",".github/workflows/ci.yml"]
}

7 rust patterns, 5 docs patterns, comments stripped correctly.

This PR touches ci.yml, which is in both filters, so its own run exercises the change end to end.

Note on the website

An earlier revision of this description claimed no job builds the website. That was wrong — Netlify does. netlify.toml runs the real Docusaurus build:

base = "website"
command = "npm run build"
ignore = "... git diff --quiet \"$CACHED_COMMIT_REF\" \"$COMMIT_REF\" -- ':(top)website' ':(top)docs'"

The ignore clause skips the build only when neither website/ nor docs/ changed, which is why Rust-only PRs show "Deploy Preview canceled" — that's the skip working as designed, not missing coverage.

So the website is covered; it's just covered by Netlify rather than by this workflow. No filter change is needed for it, and none is made here.

[netlify]

✅ Deploy Preview for angry-raman-7c44f6 canceled.

Name	Link
🔨 Latest commit	75e93e8
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