Add a class diagram for terms.

The diagram is based on the diagram created by
Graham Higgins (@gjhiggins) in https://github.com/RDFLib/rdflib/issues/1526

This shows the class heirarchy of various terms such as Identifier,
IdentifiedNode, URIRef, Literal, etc.

The diagram is in [plantuml](https://plantuml.com/class-diagram) and
compiled to svg by the
[kroki extension for sphinx](https://github.com/sphinx-contrib/kroki).

Diagrams can be rendered from the plantuml at https://kroki.io/.

Other changes:
 - Some updates to the "Writing RDFLib Documentation" page.

Add typing to rdflib.namespace

Add IdentifiedNode abstract intermediary class

Everything is as typed as it can be in here, there is maybe some
improvement that can be made to `__new__` in some places but I think
there may be some runtime changes needed there so I don't want to change
that now, the inferred return types are close enough.

Some of the type hints are based on pytest-monkeytype output which was
slightly tuned for simplicity and with some additional typing for methods
that are not being tested.

This changeset contains type changes only and does not change runtime
behaviour.

This patch adds and exposes an intermediary class `IdentifiedNode` as a
superclass of `URIRef` and `BNode`.  From review of the subclass methods
for identical implementations, two appeared to be able to move into this
superclass.

Thanks to Nicholas Car for finding this pragmatic name.

This patch addresses at least some of Issue 1526.

References:
* https://github.com/RDFLib/rdflib/issues/1526

Cc: Nicholas Car <nicholas.car@surroundaustralia.com>
Signed-off-by: Alex Nelson <alexander.nelson@nist.gov>

Bump sphinx from 4.3.2 to 4.4.0

Add a test case for a prefix followed by dot in Turtle format

Merging without second review as this is extensively tested against crafted expectations and the turtle parser which uses different code for unquoting.