fix(docs): drop PDF and EPUB builds from RTD (#49)

## Summary

RTD has been failing the build pipeline for `latest` because the PDF
format can't be produced. HTML and htmlzip succeed; PDF fails on
`pdflatex`'s inability to render the README's SVG badges or the IPA
Unicode in the project pronunciation, leaves a damaged output, and RTD's
uploader refuses with *"Build output directory for format 'pdf' contains
multiple files."*

This drops the `formats: all` line from `.readthedocs.yml`, which
defaults RTD back to HTML-only.

Re-introducing PDF would require switching the LaTeX engine to
`xelatex`/`lualatex` and hiding every README badge behind `.. only::
html` directives, with the standing risk that any new badge or non-ASCII
character re-breaks it. PDFs of a regex-builder library aren't worth
that maintenance bill.

If the htmlzip download specifically is missed, that's a one-line
follow-up: `formats: [htmlzip]`.

Closes #48

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