* feat: `clear_edges` in `networkx.Graph`

Method to clear all edges in a graph without altering nodes
or graph attributes. Added corresponding test too.

* tests: improve `test_clear` in `TestGraph`

Added a graph attribute before clearing to test its proper deletion.
too.

* tests: add tests for `clear` and `clear_edges` specific to DiGraph

To test that DiGraph-specific edge records are cleared as well.

* fix: specific `clear_edges` for `DiGraph`

So that DiGraph's pred/succ also get cleared.

* Convert docstrings tests and docs reference links to add clear_edges

Co-authored-by: Dan Schult <dschult@colgate.edu>

* Simplify the Graphview and SubGraphView system

- add tests to show that extensions of base graph classes (only add new functions)
  should be able to use the Graph.subgraph and Graph.copy methods easily
- Remove ReadOnlyGraph class in favor of existing freeze() function
- Switch all GraphView classes to generic_graph_view function
- Switch all SubGraph classes to subgraph_view function
- Introduce deprecated functions that act like the deprecated classes.

Still need to:
- add docs
- add tests
- make sure backward compatible and marked as deprecated
- remove GraphView and SubGraph construct from rest of codebase
- update release docs

Fixes #2889
Fixes #2793
Fixes #2796
Fixes #2741

* Ease subclassing for to_(un)directed

- add to_directed_class indicator functions to base classes
- start deprecating G.fresh_copy
- update function.to(un)directed

* Remove G.fresh_copy from code replace with __class__

Add deprecation warnings for GraphView classes, ReverseView and
SubGraph. Also for fresh_copy function.

Part of #1393

* Revamp tutorial.rst

* update docs and simpliy nbunch usage

* Revise base class docs

* Read-through and tweak rst files

Changes mostly due to PR #2604.

* Finish directory reorg and make minor text edits

* Remove cruft from conf.py

* Fix broken links and comply with pep8