docs: don't use kroki (#2284)

The Kroki server is currently experiencing some issues which breaks our
build, this change eliminates the use of Kroki in favour of directly
using the generated SVG images which is checked into git alongside the
PlantUML sources.

I also added a task to the Taskfile to re-generate the SVG images from
the PlantUML sources by calling docker.

Taskfile.yml

@@ -329,6 +329,19 @@ tasks:
             {{.DOCKER}} image push {{.OCI_REFERENCE}}:latest
             {{.DOCKER}} image push {{.OCI_REFERENCE}}:${_latest_rdflib_version}
           fi
+
+  docs:build-diagrams:
+    desc: Build documentation diagrams
+    cmds:
+      - cmd: |
+          shopt -s globstar;
+          for plantuml_file in ./**/*.plantuml
+          do
+            cat "${plantuml_file}" \
+              | docker run --rm -i plantuml/plantuml -tsvg -pipe \
+              > "${plantuml_file%.*}.svg"
+          done
+
   _rimraf:
     # This task is a utility task for recursively removing directories, it is
     # similar to rm -rf but not identical and it should work wherever there is

docs/_static/term_class_hierarchy.plantuml

@@ -0,0 +1,58 @@
+@startuml
+skinparam shadowing false
+skinparam monochrome true
+skinparam packageStyle rectangle
+skinparam backgroundColor FFFFFE
+
+class Node
+
+class Identifier {
+    eq(other) -> bool
+    neq(other) -> bool
+    startswith(prefix: str, start, end) -> bool
+}
+Identifier -up-|> Node
+
+class IdentifiedNode {
+    toPython() -> str
+}
+IdentifiedNode -up-|> Identifier
+
+class URIRef {
+    n3(namespace_manager) -> str
+    defrag() -> URIRef
+    de_skolemize() -> BNode
+}
+URIRef -up-|> IdentifiedNode
+
+
+class Genid
+Genid -up-|> URIRef
+
+class RDFLibGenid
+RDFLibGenid -up-|> Genid
+
+class BNode {
+    n3(namespace_manager) -> str
+    skolemize(authority, basepath) -> RDFLibGenid
+}
+BNode -up-|> IdentifiedNode
+
+class Literal {
+    datatype: Optional[str]
+    lang: Optional[str]
+    value: Any
+
+    normalize() -> Literal
+    n3(namespace_manager) -> str
+    toPython() -> str
+}
+Literal -up-|> Identifier
+
+class Variable {
+    n3(namespace_manager) -> str
+    toPython() -> str
+}
+Variable -up-|> Identifier
+
+@enduml

docs/conf.py

@@ -44,7 +44,6 @@
     "sphinx.ext.ifconfig",
     "sphinx.ext.viewcode",
     "myst_parser",
-    "sphinxcontrib.kroki",
     "sphinx.ext.autosectionlabel",
 ]
 

docs/rdf_terms.rst

@@ -17,68 +17,11 @@ Class hierarchy
 All terms in RDFLib are sub-classes of the :class:`rdflib.term.Identifier` class. A class diagram of the various terms is:
 
 .. _term_class_hierarchy:
-.. kroki::
-   :caption: Term Class Hierarchy
-   :type: plantuml
-
-    @startuml
-    skinparam shadowing false
-    skinparam monochrome true
-    skinparam packageStyle rectangle
-    skinparam backgroundColor FFFFFE
-
-    class Node
-
-    class Identifier {
-        eq(other) -> bool
-        neq(other) -> bool
-        startswith(prefix: str, start, end) -> bool
-    }
-    Identifier -up-|> Node
-    
-    class IdentifiedNode {
-        toPython() -> str
-    }
-    IdentifiedNode -up-|> Identifier
-    
-    class URIRef {
-        n3(namespace_manager) -> str
-        defrag() -> URIRef
-        de_skolemize() -> BNode
-    }
-    URIRef -up-|> IdentifiedNode
-    
-
-    class Genid
-    Genid -up-|> URIRef
-    
-    class RDFLibGenid
-    RDFLibGenid -up-|> Genid
-    
-    class BNode {
-        n3(namespace_manager) -> str
-        skolemize(authority, basepath) -> RDFLibGenid
-    }
-    BNode -up-|> IdentifiedNode
-    
-    class Literal {
-        datatype: Optional[str]
-        lang: Optional[str]
-        value: Any
-
-        normalize() -> Literal
-        n3(namespace_manager) -> str
-        toPython() -> str
-    }
-    Literal -up-|> Identifier
-    
-    class Variable {
-        n3(namespace_manager) -> str
-        toPython() -> str
-    }
-    Variable -up-|> Identifier
-    
-    @enduml
+.. figure:: /_static/term_class_hierarchy.svg
+   :alt: Term Class Hierarchy
+
+   Term Class Hierarchy
+
 
 Nodes are a subset of the Terms that underlying stores actually persist.