Docs updates for E/A#95396 Add None option for distance rate default category

[MelvinBot]

Explanation of Change

Documents the Default category setting for Distance rates in NewDot, including the new None option added in #95396, which lets a Workspace Admin clear an already-selected default category.

Before this PR, Set-distance-rates.md documented enabling, adding, editing, and removing Distance rates, but did not mention the Default category setting at all. This adds:

• A How to set a Default category for Distance rates section covering how to set a default category and how to select None to clear it.
• A matching FAQ entry: Can I remove the Default category for Distance rates?

Related

• App PR being documented: [Payment due @parasharrajat] Add None option to clear default custom unit category #95396

UI labels verified against code

All labels were verified against src/languages/en.ts and the Distance rates settings page:

• Default category (workspace.common.defaultCategory → 'Default category')
• None (common.none → 'None')
• Settings (common.settings → 'Settings')

Note: This was authored from static code analysis only — browser/live-UI verification was unavailable in this run, so the exact navigation and label placement should be confirmed against the live app before merging.

Fixed Issues

$ Docs update for #95396

@ShridharGoel, please review the files changed and confirm they reflect the current behavior. Then mark this PR Ready for review.