Markdown check

[steve-fenton-octopus]

Adds a Markdown Lint check to pull requests.

This only runs against changed files, so contributors only need to fix markdown issues in files they add or change.

When everything is fine:

markdownlint-cli2 v0.20.0 (markdownlint v0.40.0)
Finding: src/pages/docs/octopus-ai/recovery-agent/index.mdx
Linting: 1 file(s)
Summary: 0 error(s)

When there's a markdown issue:

Error: _shared/in-this-section.md:2 MD012/no-multiple-blanks Multiple consecutive blank lines [Expected: 1; Actual: 2] https://github.com/DavidAnson/markdownlint/blob/v0.40.0/doc/md012.md
Error: _shared/in-this-section.md:3 MD041/first-line-heading/first-line-h1 First line in a file should be a top-level heading [Context: "## In this section"] https://github.com/DavidAnson/markdownlint/blob/v0.40.0/doc/md041.md

[samwld]

LGTM