Threadfin: New role - Githubissues

Please describe the purpose of this Pull Request.

New Threadfin role.

For new Sandbox app/role documentation, please confirm you have completed the following tasks:

[x] App documentation page created
[x] markdownlint reports no errors on edited page(s) - use https://dlaa.me/markdownlint/ if your IDE does not have this built in
[x] Reference added in docs/sandbox/index.md
[x] Reference added in mkdocs.yml nav menu

saltyorg / docs