Add a docstring

Each function of the IGitt library should have the description of the function, including its parameters.

Docstring including params details should be added to the following functions.

• Content - init, get_content, update
• Hoster - get_repo
• Installation - init
• Issue - create
• Notification - init, fetch_all
• Organization - init, create
• Repository - create_issue, create_fork, create_merge_request, create_file, _search_in_range, search_mrs, search_issues, get_permission_level
• Team - init, is_member
• User - installed_repositories

Changes should be done in functions of GitHub, GitLab, Interfaces Classes. For example if you to need add docstring to create function of Issue, you should change it in GitHubIssue, GitLabIssue, Issue.