New gitlab templates

Description

Updates this template and adds a new documentation issue template.

Changes

• This template no longer requires 2/4 reviewers since we will use GitLab's built-in features.
• This template now requires up-to-date documentation and @Nullable where possible

Additions

• A new documentation issue template suspiciously similar to the feature issue template

Test and Review

To be filled in by the reviewers

• All of the methods are commented to expectation

• The methods are tested to satisfaction

• There are no unnecessary files added to the PR

• The continuous integration has no problems with the PR

• The PR is filled in as requested e.g. labels added reviews requested

• At least two members have reviewed this PR (4 in case of master branch)