Formatting fixes for "Relink Don't Rebuild" (25H2) (#357)

* update `mdbook`: 0.4.40 → 0.4.52

0.4.48 changes how footnotes are rendered (more in line w/GFM):
https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-0448

* rdr: use `mdbook`'s `class="warning"` instead of GFM alert

([`mdbook-alerts`](https://github.com/lambdalisue/rs-mdbook-alerts)
exists but since we've only got the one usage...)

* add `mdbook-mermaid`

note: running `mdbook-mermaid install` before building the book rather
than vendoring `mermaid.min.js`/`mermaid-init.js` into the repo

Author: rrbutani

.github/workflows/mdbook.yml

@@ -33,14 +33,16 @@ jobs:
   build:
     runs-on: ubuntu-latest
     env:
-      MDBOOK_VERSION: 0.4.40
+      MDBOOK_VERSION: 0.4.52
+      MDBOOK_MERMAID_VERSION: 0.15.0
     steps:
       - uses: actions/checkout@v4
-      - name: Install mdBook
+      - name: Install mdBook + preprocessors
         run: |
           curl --proto '=https' --tlsv1.2 https://sh.rustup.rs -sSf -y | sh
           rustup update
           cargo install --version ${MDBOOK_VERSION} mdbook
+          cargo install --version ${MDBOOK_MERMAID_VERSION} mdbook-mermaid
       - uses: extractions/setup-just@v2
       - name: Setup Pages
         id: pages

.gitignore

@@ -1,4 +1,7 @@
 book
 target
 .cache
-src/api
+src/api
+
+# copied into the repo by `mdbook-mermaid` install:
+/mermaid*.js