Following the single docs codebase change, all internal links
should be relative. Also cleans up one table

Moves every fixture source file from
spec/javascripts to spec/frontend

Introduces frontend:fixtures rake task to replace
karma:fixtures and update documentation and CI scripts
to match

Remove unneeded trailing whitespace in lines in
/development /ci /api and /administration docs.

Fixing square brackets, links, etc in:
  merge_requests.md
  vulnerabilities.md
  issues.md
  issues_statistics.md
  pipelines.md
  services.md
  group_milestones.md
  milestones.md
  runners.md
  changelog.md
  issue_workflow.md
  elasticsearch.md
  api_graphql_styleguide.md
  automatic_ce_ee_merge.md
  file_storage.md
  architecture.md
  database_debugging.md
  index.md
  index.md
  frontend_testing.md
  pry_debugging.md
  vue.md
  development_process.md