[Documentation] PSR12 - File Header

[dingo-d]

The PR contains the documentation for the PSR12/Files/FileHeader sniff.

Description

This PR will add the documentation for the above-mentioned sniff, according to the official standard definitions.

Suggested changelog entry

Add documentation for the PSR12 FileHeader sniff

Related issues/external references

Part of #148

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
  • This change is only breaking for integrators, not for external standards or end-users.
• Documentation improvement

PR checklist

• I have checked there is no other PR open for the same change.
• I have read the Contribution Guidelines.
• I grant the project the right to include and distribute the code under the BSD-3-Clause license (and I have the right to grant these rights).
• I have added tests to cover my changes.
• I have verified that the code complies with the projects coding standards.
• [Required for new sniffs] I have added XML documentation for the sniff.

[jrfnl]

@dingo-d Thank you for working on this! My apologies for taking a while before I got to reviewing the PR.

After reviewing this PR, these are my findings:

1. While the code samples illustrate the rules and sort of list them in the "Valid" titles, the actual rules themselves are not explained in a <standard> element/elements.
   "Ensures that the PHP file header is properly formatted." is non-descript and doesn't make it clear what "properly formatted" means.
2. The docs talk about "header blocks" but don't actually explain what code is considered part of the file header. I.e. what is a "header block" ? This makes the docs unclear.
   I think the list of header blocks as per PSR12 will need to be made explicit in the docs.
3. The docs only contain examples for three out of the four rules the sniff checks for.
   The "Similar statements must be grouped together inside header blocks" rule is missing.
4. Please review the used code sample for correct use of <em> tags in all relevant places.

[jrfnl]

Closing - see: #148 (comment)