Many small fixes to api docs which were merged from EE to CE,
and tables cleaned up, as noted in issue
https://gitlab.com/gitlab-org/gitlab-ce/issues/64072
Add env_var and file as supported types for CI variables. Variables of
type file expose to users existing gitlab-runner behaviour - save
variable value into a temp file and set the path to this file in an ENV
var named after the variable key.
Resolves https://gitlab.com/gitlab-org/gitlab-ce/issues/46806.
