Exclude node_modules subfolder for lint-markdown.sh (#543)

## Ticket

Resolves [lint-markdown.sh](https://github.com/navapbc/template-infra/blob/main/bin/lint-markdown.sh)
unnecessarily analyzes `md` files under `node_modules/`.

## Changes

Exclude files under `*/node_modules/` from being analyzed by adding the
arguments `-not -path "*/node_modules/*"`.

## Context for reviewers

The `gaurav-nelson/github-action-markdown-link-check` action [ignores
node_modules
files](https://github.com/navapbc/bid-fed-va-spruce-challenge-practice3/actions/runs/7576561682/job/20635654502?pr=4#step:4:109).

## Testing

- Instantiate the `infra` and `nextjs` templates into a single repo.
- Run `npm install` for the nextjs application so that a `node_modules`
subfolder is populated
- Run `bin/lint-markdown.sh` and ensure the `md` files under the
`node_modules` subfolder are not analyzed

bin/lint-markdown.sh

@@ -13,6 +13,6 @@ cd "${SCRIPT_DIR}/.." >/dev/null || exit 1
 
 LINK_CHECK_CONFIG=".github/workflows/markdownlint-config.json"
 
-# Recursively find all markdown files (*.md) in this directory. Pass them in as args to the lint
-# command using the handy `xargs` command.
-find . -name \*.md -print0 | xargs -0 -n1 npx markdown-link-check --config $LINK_CHECK_CONFIG
+# Recursively find all markdown files (*.md) in the current directory, excluding node_modules subfolders.
+# Pass them in as args to the lint command using the handy `xargs` command.
+find . -name \*.md -not -path "*/node_modules/*" -print0 | xargs -0 -n1 npx markdown-link-check --config $LINK_CHECK_CONFIG