Remove some unneeeded sphinx-asdf directives

[braingram]

Description

Remove use of 2 sphinx-asdf directives:

• asdf : replaced with inline copies of produced files
• runcode : replaced with code blocks (executed as doctests)

Neither of the above is used outside of asdf so once we merge this we can remove those directives from sphinx-asdf.

To get the doctests to behave locally I needed to:

• introduce a tmp_cwd fixture for doctests to run them in a temporary directory
• rename the docs/asdf folder to docs/_asdf to avoid import confusion where doctests were trying to import docs/asdf as if it were an asdf module

Here are some of the changed pages:

• overview:
  • https://asdf-website--2057.org.readthedocs.build/projects/asdf/en/2057/_asdf/overview.html#hello-world
  • https://www.asdf-format.org/projects/asdf/en/latest/asdf/overview.html#creating-files
• converter block storage:
  • https://asdf-website--2057.org.readthedocs.build/projects/asdf/en/2057/_asdf/extending/converters.html#block-storage
  • https://www.asdf-format.org/projects/asdf/en/latest/asdf/extending/converters.html#block-storage
• features:
  • https://asdf-website--2057.org.readthedocs.build/projects/asdf/en/2057/_asdf/features.html#external-references
  • https://www.asdf-format.org/projects/asdf/en/latest/asdf/features.html#external-references
• arrays:
  • https://asdf-website--2057.org.readthedocs.build/projects/asdf/en/2057/_asdf/arrays.html#saving-arrays
  • https://www.asdf-format.org/projects/asdf/en/latest/asdf/arrays.html#saving-arrays

pytestdev failure is unrelated: #2055

Tasks

• run prek on your machine
• run pytest on your machine
• Does this PR add new features and / or change user-facing code / API? (if not, label with no-changelog-entry-needed)
  • write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see below for change types)
  • update relevant docstrings and / or docs/ page
  • for any new features, add unit tests

news fragment change types...

• changes/<PR#>.feature.rst: new feature
• changes/<PR#>.bugfix.rst: bug fix
• changes/<PR#>.doc.rst: documentation change
• changes/<PR#>.removal.rst: deprecation or removal of public API
• changes/<PR#>.general.rst: infrastructure or miscellaneous change