Missing mandatory trailer in commit message format in docs

[rdimaio]

Here: https://rucio.github.io/documentation/contributing/#4-commit-your-changes

Under "Commit Message Format:", the format is described as:

<type>(<scope>): <description>

[optional body]

[optional footer(s)]

However, later we say that "All commits must reference a GitHub issue using git trailers. This ensures proper traceability and automatic issue closure." and provide this commit example:

feat(Transfers): Group bulk transfers by authentication method

The authentication method is a property of the FTS job. If Rucio does
not partition the transfers based on it, then some may be done with an
incorrect authentication method.

Closes: #8199

So I think the original commit message format should mention that there is a required trailer

[0xquark]

Related: #728

This issue is part of the second iteration of the Conventional Commits update. I wanted to wait a week to collect feedback before implementing these changes.