Add Markdown documentation linting

[msteslov]

Fixes #1785.

Adds a Markdown linting job to the basic CI workflow using markdownlint-cli2.

Testing:

• npx --yes markdownlint-cli2@0.22.1 --config .markdownlint.yml "**/*.md" "#build" "#**/build"
• actionlint -shellcheck "" .github/workflows/basic.yml

• Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
• Does this PR change the list of algorithms available -- either adding, removing, or renaming? Does this PR otherwise change an API? (If so, PRs in fully supported downstream projects dependent on these, i.e., oqs-provider will also need to be ready for review and merge by the time this is merged. Also, make sure to update the list of algorithms in the continuous benchmarking files: .github/workflows/kem-bench.yml and sig-bench.yml)

AI assistance was used for repository inspection and drafting; I reviewed and verified the final changes.

[xuganyu96]

LGTM. Thank you for the contribution.

[coveralls]

coverage: 82.263% (-0.004%) from 82.267% — msteslov:add-markdown-linter into open-quantum-safe:main

[xuganyu96]

I am sorry to have to walk back on my previous approval. The code change makes sense to me, but the choice of tool could use some justification. I did a quick search and found other markdown linters written in Ruby (markdownlint), Python (pymarkdown), and Rust (rumdl), and many others. It feels a tiny bit uncomfortable to introduce NPM into dev dependencies. I intuitively prefer keeping dev dependencies either in the Python ecosystem or as small binaries. Of course if markdownlint-cli is indeed a much more mature project than the other choices, then I am okay with introducing it into liboqs.