add static/robots.txt, disabling crawling of /api/
add static/robots.txt, disabling crawling of /api/

This is related to T662, but doesn't completely addresses it as we also need an
HTTP-level alias /robots.txt -> /static/robots.txt.

SoftwareHeritage/swh-web-ui,SoftwareHeritage/swh-web-ui,SoftwareHeritage/swh-web-ui