Outlining the difference between Allure severity levels

[sta1r]

This PR attempts to clarify the difference between the different levels of Severity (MINOR, AVERAGE, MAJOR, CRITICAL or BLOCKER), as per Magento's customised use of this Allure annotation.

Description

When writing MFTF tests, it's often difficult to know what severity level to apply. Currently, the docs don't explain this.

I've learned that Magento has its own custom set of levels that are mapped to Allure's level names. I haven't linked to Allure's site or provided a table indicating the matching of Magento names to Allure names, because I don't think this is very helpful. I've just edited Alex Kolesnyk's explanation to me of the levels vis a vis their impact on conversion rate.

Broken links to devhub.io are replaced with github.com paths.

Affected pages

• https://devdocs.magento.com/mftf/docs/test/annotations.html

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/verification tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)
• Changes to Framework doesn't have backward incompatible changes for tests or have related Pull Request with fixes to tests

[coveralls]

Coverage remained the same at 56.074% when pulling c996fb4 on sta1r:AM/severity-levels into ef5be1d on magento:develop.

The base branch was changed.

The base branch was changed.

[tomreece]

Approved by @dobooth also.