feature: add fd.clean_excel() companion to fd.clean_csv() (#19)

[JohnnyWilson16]

Problem

#19 requests a one-line fd.clean_csv(). Excel exports are just as common a "messy source" and share the same shape, so an fd.clean_excel() companion is a natural follow-up.

Depends on #19 — the clean_csv API decision should land first so the two stay consistent. Labeled needs maintainer decision for that reason.

Expected outcome

fd.clean_excel(path, ...) mirroring fd.clean_csv, reusing the same cleaning path, returning cleaned data (+ optional report).

Relevant files

• src/freshdata/api.py
• docs (quickstart / api-reference)
• tests/

Acceptance criteria

• Round-trips a small .xlsx fixture through cleaning.
• API signature matches clean_csv (confirmed against [Enhancement - Junior Level] Add fd.clean_csv() for one-line CSV cleanup #19).
• Documented and tested; fast lane green.

Difficulty / skills

🚀 junior–mid — pandas IO (read_excel), API design.