Review and extend the documentation, and check it in CI

A review of all documentation pages found factual errors, dead links,
missing cross-references, and gaps in examples. This fixes them and adds
checks so the same problems are caught automatically.

Fixes:
- wrong signatures and version histories (operator!= C++20 member,
  binary() subtype type, get<PointerType>(), JSON_NO_THREAD_LOCAL, ...)
- stale descriptions (number parsing since #5283, UBJSON table, SAX
  example that no longer compiled, tsl::ordered_map advice)
- dead internal and external links; repology.org badges (the domain is
  suspended) replaced by badges that query the registries directly
- deprecation notes link the migration guide; the guide itself fixed

Additions:
- "See also" sections, cross-references, 25 runnable examples, 12
  Mermaid diagrams, new API pages for json_pointer::operator<=> and
  byte_container_with_subtype::operator==/!=
- landing page, guides for untrusted input and performance
- "unreleased" badge after versions newer than the latest release

Checks:
- strict documentation build (broken links/anchors fail it); CI and
  the publish workflow fetch the full history the build needs
- weekly external link check, Mermaid syntax check in CI
- check_structure.py: example titles, heading levels, alt texts,
  header links, docset index coverage; its unused-example check works
  again
- all examples produce the same output on every platform

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

.github/workflows/check_docs_links.yml

@@ -0,0 +1,46 @@
+name: Check documentation links
+
+# check the links of the documentation weekly; external links break without any change in this repository
+on:
+  schedule:
+    - cron: '17 4 * * 1'
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ${{ github.workflow }}
+  cancel-in-progress: true
+
+jobs:
+  check_docs_links:
+    if: github.repository == 'nlohmann/json'
+    runs-on: ubuntu-latest
+    timeout-minutes: 30
+    steps:
+      - name: Harden Runner
+        uses: step-security/harden-runner@e14015d583714f6e62063499dc959a02595150a1 # v2.21.1
+        with:
+          egress-policy: audit
+
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          persist-credentials: false
+
+      - name: Install virtual environment
+        run: make install_venv -C docs/mkdocs
+
+      - name: Check links
+        shell: bash  # adds -o pipefail, so tee does not hide the exit code
+        run: make link_check -C docs/mkdocs 2>&1 | tee link_check.log
+
+      - name: Summarize broken links
+        if: failure()
+        run: |
+          {
+            echo '### Broken documentation links'
+            echo '```'
+            grep 'invalid url' link_check.log || true
+            echo '```'
+          } >> "$GITHUB_STEP_SUMMARY"

.github/workflows/publish_documentation.yml

@@ -31,6 +31,10 @@ jobs:
           egress-policy: audit
 
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          # the git-revision-date-localized plugin needs the history for correct "last update" dates and so the
+          # strict build does not fail on a shallow clone
+          fetch-depth: 0
 
       - name: Install virtual environment
         run: make install_venv -C docs/mkdocs

.github/workflows/ubuntu.yml

@@ -389,7 +389,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        target: [ci_test_examples, ci_test_build_documentation]
+        target: [ci_test_examples, ci_test_build_documentation, ci_test_documentation_mermaid]
     steps:
       - name: Harden Runner
         uses: step-security/harden-runner@e14015d583714f6e62063499dc959a02595150a1 # v2.21.1
@@ -399,6 +399,12 @@ jobs:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           persist-credentials: false
+          # the git-revision-date-localized plugin needs the history; a shallow clone makes the strict build fail
+          fetch-depth: ${{ matrix.target == 'ci_test_build_documentation' && '0' || '1' }}
+      - uses: actions/setup-node@820762786026740c76f36085b0efc47a31fe5020 # v7.0.0
+        if: matrix.target == 'ci_test_documentation_mermaid'
+        with:
+          node-version: 24
       - name: Run CMake
         run: cmake -S . -B build -DJSON_CI=On
       - name: Build