Add core maintainers section to package READMEs

[carbolymer]

Description

Adds a "Core maintainers" section to the cardano-node-chairman, cardano-submit-api and cardano-testnet READMEs, matching the sections merged in IntersectMBO/cardano-api#1347 and IntersectMBO/cardano-cli#1446.
cardano-node-chairman had no README at all, so it gets a short one describing what the tool does.

Each maintainer name carries the BlockPQR logo linking to https://blockpqr.io.
The logo is a <picture> element wrapped in a markdown link.
It switches between two SVG assets committed under .github/: teal light art for the dark theme and deep green dark art for the light theme.
Both files come from the BlockPQR media kit, with their viewBox cropped to the artwork's bounding box, because the original exports carry large transparent margins that made the logo render small and misaligned next to text.
The image paths start with ../.github/ because the READMEs live one directory below the repository root.

This is a docs-only change.
To review, open the rendered READMEs on this branch in both GitHub themes: each maintainer line should show a 14px BlockPQR lockup linking to blockpqr.io.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. These may include:
  • golden tests
  • property tests
  • roundtrip tests
  • integration tests
    See Running tests for more details
• Any changes are noted in the CHANGELOG.md for affected package
  • cardano-node-chairman, cardano-submit-api and cardano-testnet instead need a
    changelog fragment in <package>/.changes/, because their CHANGELOG.md is generated
    from fragments at release time. Copy _TEMPLATE.yml from that directory, or run
    nix run github:input-output-hk/cardano-dev#herald -- new
• The version bounds in .cabal files are updated
• CI passes. See note on CI. The following CI checks are required:
  • Code is linted with hlint. See .github/workflows/check-hlint.yml to get the hlint version
  • Code is formatted with stylish-haskell. See .github/workflows/stylish-haskell.yml to get the stylish-haskell version
  • Code builds on Linux, MacOS and Windows for ghc-9.6 and ghc-9.12
• Self-reviewed the diff

Note on CI

If your PR is from a fork, the necessary CI jobs won't trigger automatically for security reasons.
You will need to get someone with write privileges. Please contact IOG node developers to do this
for you.