Update docs to markdown style guide.

CONTRIBUTING.md

MAINTENANCE.md

PROCESS.md

README.md

doc/README.md

doc/api/README.md

doc/api/branches.md

doc/api/commits.md

doc/api/deploy_keys.md

doc/api/issues.md

doc/api/merge_requests.md

doc/api/milestones.md

doc/api/project_snippets.md

doc/api/repository_files.md

doc/api/system_hooks.md

doc/api/users.md

doc/development/README.md

doc/development/architecture.md

doc/development/shell_commands.md

doc/install/README.md