docs: repoint nine dead source links in the onboarding guide - #4025

Closes #4024.

• ran the linter to address style issues (tox -e fix)
  — not run: I could not install a working tox in this environment. Instead I checked
  the rules tox -e fix would enforce on an RST-only diff by hand: no line introduced by
  this change exceeds 120 characters (verified by script — the only lines over 120 in the
  file are the four that were already over on main), and the RST parses under docutils.
  Please re-run tox -e fix on my branch; I'll amend if it objects.
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
  — not applicable: the change contains no code. The equivalent check is the link
  resolution described above (67 links in docs/, 0 unresolved), which I ran before and
  after. If you would find a repo-level guard against this class of regression useful, I'd
  be glad to contribute one as a separate PR rather than smuggle it in here.
• added news fragment in docs/changelog folder — docs/changelog/4024.doc.rst
• updated/extended the documentation — this PR is the documentation change

Problem

Nine blob/main source links in docs/onboarding.rst point at paths that do not exist in
main, so they render as GitHub 404s. Six of them additionally label the link with a class or
module name that no longer exists, so the prose is misleading independently of the dead link.

This matters more than it would in most files because onboarding.rst is what a new
contributor reads to learn where things live — these are exactly the links someone with no
prior knowledge of the layout will click.

How it was reproduced

I resolved every https://github.com/tox-dev/tox/blob/main/<path> link in docs/ against the
working tree at main, rather than checking them over HTTP (GitHub rate-limits that hard enough
to produce false positives in both directions):

links resolving to an existing file: 58
links pointing at a NON-EXISTENT path: 9

All nine were in onboarding.rst; the other 58 were already correct and are untouched.

What the fix does

Each link was resolved by locating the definition it refers to, not by guessing at a
similar-looking filename:

The tox.toml → TomlTox and pyproject.toml → TomlPyProject split was confirmed against
the import order in src/tox/config/source/discover.py so the two are not transposed.

Documentation only — no source changes, and no links outside the nine broken ones were touched.

How it was verified

• Re-ran the same tree resolution after the change: 67 blob links in docs/, 0 broken.
• Confirmed each of the nine new targets parses into a real reference node via docutils
  (80 reference nodes in the file), so none of them silently became plain text — relevant
  because two of the fixed links wrap across lines.
• Checked the 120-character rule from docs/development.rst. Two of my edits initially pushed
  their lines to 122 and 123, so I reflowed them: the show_config cell now wraps using the
  same 8-space continuation style already used by the sequential.py cell in that same
  list-table. Lines over 120 in the file are now exactly the four that were already over on
  main (5, 72, 410, 680) — the change adds none.
• Verified the class and module names by grepping for their definitions
  (LocalSubProcessExecuteInstance at local_sub_process/__init__.py:162,
  LocalSubprocessExecuteStatus at :62, IniLoader at loader/ini/__init__.py:35,
  TomlLoader at loader/toml/__init__.py:31, CliEnv at session/env_select.py:41).

I could not run tox -e docs in this environment, so the RST was validated with docutils
rather than a full Sphinx build. The change is confined to link targets and link labels inside
existing constructs, so I would not expect a Sphinx-specific failure, but please flag it if the
docs build disagrees.

Changelog entry added as docs/changelog/4024.doc.rst. If you would rather keep the abstract
ExecuteInstance / ExecuteStatus names in the prose at lines 447/450 and only repoint the
URLs, say so and I'll amend.

Disclosure: this change was prepared with AI assistance. The link resolution, the docutils
reference-node check and the line-length comparison described above were actually run, and I
have reviewed every one of the nine mappings against the class definitions in the tree.
Flagging it so reviewers can weigh it as they see fit.