Added examples and modified url links

[ajithkumar-maragathavel]

Description

This PR adds example code snippets for the assert nodes and also updated the invalid external URL links with the correct URLs.

Affected Pages

1. https://devdocs.magento.com/mftf/docs/test/assertions.html
2. https://devdocs.magento.com/mftf/docs/test.html

Contribution checklist

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

[jilu1]

@magento import pull request to https://github.com/magento-commerce/magento2-functional-testing-framework

[magento-engcom-team]

@jilu1 the pull request successfully imported.

[jilu1]

@ajithkumar-maragathavel Thank you for your PR! We are processing this PR and contents will be in release 3.5.0.