fix(docs): drop PDF and EPUB builds from Read the Docs

RTD's PDF format was failing because:

1. README.rst's SVG badges can't be embedded by pdflatex
   (`Unknown graphics extension: .svg` x 8).
2. The IPA pronunciation `/ˈɛdɪfaɪ/` uses Unicode codepoints not
   set up for use with pdflatex (would need xelatex/lualatex).

`latexmk` exited 12, leaving a damaged `edify.pdf` plus aux files in
the output directory; RTD's uploader then refused with "Build output
directory for format 'pdf' contains multiple files."

Removing `formats: all` from `.readthedocs.yml` defaults RTD to HTML
only. PDFs of a regex-builder library aren't worth the maintenance
cost — fixing them would require switching the LaTeX engine and
hiding every badge behind `.. only:: html` directives, with the
guarantee that any new badge or Unicode glyph re-breaks it.

Closes #48

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