Add a citation manual

Add a citation.md file that contains the citation guide. Then, to add the url of the raw file to the tskit-dev website's citation page: https://tskit.dev/citation/, add this url to a new field "citation_md" in the _software/tskit.md file in the tskit-site repo

example: 
`citation_md: https://raw.githubusercontent.com/pyslim/tskit/refs/heads/main/docs/citation.md
`