Add pkg-config to Markdown linting image. Required for some projects

What does this MR do and why?

When making use of the linting image from !4702 (merged) to gitaly project, it became apparent we'd need pkgconfig (https://pkgs.alpinelinux.org/package/v3.19/main/x86_64/pkgconf) Alpine package to have it run without an error: https://gitlab.com/gitlab-org/gitaly/-/jobs/6622484216.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. On main branch, run docker buildx build --file dockerfiles/gitlab-docs-lint-markdown.Dockerfile . --platform=linux/amd64 --build-arg="ALPINE_VERSION=3.19" --build-arg="VALE_VERSION=3.4.1" --build-arg="MARKDOWNLINT2_VERSION=0.13.0" --build-arg="LYCHEE_VERSION=0.14.3".
3. Run docker run --rm -it --platform=linux/amd64 <Docker image SHA> bash.
4. In the running Docker container, run:
   1. git clone https://gitlab.com/gitlab-org/gitaly.git.
   2. cd gitaly.
   3. make lint-docs. You should get the error: /bin/bash: line 1: pkg-config: command not found.
5. Check out this branch.
6. On the eread/add-pkg_config-to-markdown-linting-image branch, run docker buildx build --file dockerfiles/gitlab-docs-lint-markdown.Dockerfile . --platform=linux/amd64 --build-arg="ALPINE_VERSION=3.19" --build-arg="VALE_VERSION=3.4.1" --build-arg="MARKDOWNLINT2_VERSION=0.13.0" --build-arg="LYCHEE_VERSION=0.14.3".
7. Run docker run --rm -it --platform=linux/amd64 <Docker image SHA> bash.
8. In the running Docker container, run:
   1. git clone https://gitlab.com/gitlab-org/gitaly.git.
   2. cd gitaly.
   3. make lint-docs. You should get no error.

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.