Add Odometer distance expense to help site docs

[MelvinBot]

Summary

• Adds documentation for the new Odometer distance expense type to Distance-Expenses.md and Create-an-Expense.md
• The Odometer feature was released in Enable Odometer distance expense feature #86769 (beta flag removed), so the help site now needs to reflect this as a 4th distance expense option alongside GPS, Map, and Manual

Changes

• Distance-Expenses.md: Added "How to create an Odometer Distance expense" section with full step-by-step instructions; updated intro, metadata, and keywords
• Create-an-Expense.md: Added brief "How to create a Distance expense from an Odometer" section; updated intro paragraphs, metadata, and keywords

Test plan

• Verify the new Odometer sections render correctly on the help site
• Verify UI labels (Odometer, Start reading, End reading, Save for later) match the live product
• Verify cross-links still work

Co-authored-by: Julesssss 10736861+Julesssss@users.noreply.github.com

[Julesssss]

Not sure how this was created, but it's already prepared here.