[3006.x] Rework pillar/renderers/encryption documentation

[dwoz]

What does this PR do?

Rewrites the pillar / renderers / encryption documentation to close
twelve open documentation issues in one pass. Each documented example
is also pinned by a regression test so future edits cannot silently
break the snippet a user copy-pastes from the docs.

Issues addressed

• [Docs] pillar documentation update #56239 (Pillar documentation update)
• [DOCS] Example of per-saltenv gitfs root setting #58929 (Per-saltenv gitfs/git_pillar root example)
• [DOCS] Create includes note admonition in documentation referrencing GPG keys: GPG is used for all OpenPGP/PGP/GPG related material and implementations. #59539 (GPG terminology note)
• [DOCS] default=<class 'KeyError'> in pillar.get #60378 (pillar.get default=<class 'KeyError'>)
• [DOCS] clarify pillar datastructure of underlying yaml #61283 (Clarify the pillar data structure)
• [DOCS] document Module.run usage with pyobjects renderer #61828 (Module.run with pyobjects + dotted function names)
• [DOCS] Pillar Encryption should mention shebang renderers #62733 (Shebang renderers for pillar encryption)
• [DOCS] Confusion over running custom modules in pillar #62802 (Custom modules in pillar -- master-side sync)
• [DOCS] Add how to target on pillars from specific pillarenv #63110 (Targeting -I against a specific pillarenv)
• [DOCS]Documentation py renderer needs upgrade #63698 (py renderer documentation needs upgrade)
• [DOCS] Homedir path in SALT.RENDERERS.GPG (ENCRYPTING DATA) are missing in few examples - bash/awk/perl  #65682 (--homedir missing from documented gpg encrypt examples)
• [DOCS] Pillar include subkey separator not documented #66622 (Pillar include: subkey separator)

Where

Doc / docstring changes:

• doc/topics/pillar/index.rst: new "Pillar Data Structure" section
  with worked merge example, GPG-terminology note, per-file shebang
  renderer subsection, include: key: separator note, custom modules
  subsection.
• doc/topics/targeting/pillar.rst: new "Targeting against a specific
  pillarenv" section.
• salt/modules/pillar.py: get() docstring explains the
  <class 'KeyError'> rendering of the internal sentinel.
• salt/renderers/gpg.py: all encrypt examples now pass
  --homedir /etc/salt/gpgkeys.
• salt/renderers/py.py: three new worked examples
  (__salt__["http.query"], result-based control flow, highstate
  return shape).
• salt/renderers/pyobjects.py: new subsection for Module.run with
  dotted function names via **kwargs.
• salt/pillar/git_pillar.py: new "Per-saltenv root" section before
  all_saltenvs.

Tests:

• tests/pytests/functional/pillar/test_documented_examples.py
• tests/pytests/functional/pillar/test_gpg_encryption_documented.py
• tests/pytests/unit/renderers/test_pyobjects_documented.py
• tests/pytests/unit/renderers/test_py_renderer_documented.py

Test plan

• sphinx-build -W -b html doc/ doc/_build/html -- clean.
• pre-commit run on every touched file -- Lint Salt + Lint Tests
  pass.
• pytest --run-slow over the four new test modules -- 11/11
  pass on 3006.x, 3007.x, 3008.x, master.
• GPG round-trip generates a fresh keypair in a tmp --homedir,
  uses the documented encrypt command line, and decrypts via the
  pillar gpg renderer (both decrypt_pillar and #!yaml|gpg
  shebang variants).