Ship a shared .markdownlint-cli2.jsonc so derived repos lint consistently (IDE vs CLI)

[ptr727]

Summary

ProjectTemplate's workspaces recommend the davidanson.vscode-markdownlint
extension, but the template ships no markdownlint configuration. As a result,
every derived repo gets inconsistent lint behavior: the IDE extension and any
command-line / CI run of markdownlint-cli2 apply different rule sets, and each
downstream repo has to (re)invent its own config from scratch.

Evidence

• DotNet.code-workspace:95 and Python.code-workspace:95 both list
  "davidanson.vscode-markdownlint" under recommended extensions.
• There is no .markdownlint-cli2.jsonc / .markdownlint.json / .markdownlintrc
  anywhere in the template (root or per-language project dirs).

Why this causes drift

markdownlint-cli2 and the davidanson VS Code extension both read a
.markdownlint-cli2.jsonc config file. With none present:

• The extension falls back to its bundled defaults (or a developer's personal
  user-settings config), while a CLI/CI invocation falls back to the tool
  defaults — they don't have to match.
• Style-heavy docs trip default rules that the maintainer intentionally does not
  enforce. Concretely, in a derived repo (ptr727/KiCadLibrary) a CLI run
  reported 46 MD013/line-length errors on a README.md that the IDE showed
  as clean, purely because no shared config disabled MD013.
• Each repo solves this independently. ptr727/homeassistant-purpleair already
  carries a hand-tuned .markdownlint-cli2.jsonc; KiCadLibrary had none until I
  copied that file in. That's exactly the kind of per-repo divergence the
  template exists to prevent.

Recommendation

Ship a single source-of-truth .markdownlint-cli2.jsonc at the template root
(and in each per-language project dir if those are templated independently).
Because the one file is honored by both the extension and the CLI/CI, the IDE and
the command line stay in lock-step. Suggested content (mirrors the de-facto
standard already used in homeassistant-purpleair):

{
    "config": {
        // Prose paragraphs and data-heavy tables/URLs are intentionally long;
        // reflowing at 80 cols hurts readability and churns diffs.
        "MD013": false,
        // Inline HTML is used for reference-link section dividers.
        "MD033": false,
        // Require fenced code blocks over the legacy 4-space-indented style.
        "MD046": { "style": "fenced" },
        // Wide tables are intentional where wrapping cells breaks GitHub rendering.
        "MD060": false
    },
    "gitignore": true
}

Optionally, add a one-line note to CODESTYLE.md / AGENTS.md stating that the
authoritative lint result is what this config produces (so contributors don't
"fix" rules the project has deliberately turned off), and wire
markdownlint-cli2 into the lint task / CI so the gate matches the IDE.