Update Git standards to remove optional description merging PRs

Description:

As per our current Git coding standards, when merging Pull Requests, we are required to do a "Squash and merge", but no further details are added in this process.

In practice, we also ensure the following during "Squash and merge" (in GitHub atleast):

1. Description should match PR title since that is shown in commit history
2. PR number should be there (ex: #59) in the description
3. Remove everything from optional extended description

It would be good if we can add it to the documents, if this is a step that is normalised across both GitLab and GitHub.