Add links to references of fare_leg_rules.txt

[isabelle-dr]

References to GTFS files within the documentation are linked directly to the appropriate file.

This PR adds links for instances of fare_leg_rules.txt (as well as a couple of fare_products.txt and fare_media.txt) that didn't have a link.

Type of change

GTFS Schedule

• Functional Change
• Non-Functional Change
• Documentation Maintenance

GTFS Realtime

• Specification Change
• Specification Change (Experimental Field)

Checklist

• I have read and understood the GTFS Change process.
• I have read and understood the GTFS Guiding Principles.
• I understand the Role of Advocate and the Responsibilities attached to it.
• I have signed the Contributor License Agreement (CLA).
• I have ensured that this proposal is not covered by an existing Pull Request.

[isabelle-dr]

I also noticed a few instances where file names were formatted as code while others were not, so I removed the code formatting from all file references to keep things consistent.

[Sergiodero]

Thanks, @isabelle-dr! Since this is purely documentation maintenance, we’ll keep the PR open for an additional week to allow for review before merging, following the governance process for this type of change.