Prepare release 1.0.0

[nichtich]

In addition to open issues this requires:

• [x] align pgraphs and include note about standard compliance
• [x] align parser in pg-formatter
  • [ ] some open issues still
• [x] extend and align syntax-highlighting for CodeMirror used in pg-formatter (https://github.com/pg-format/pg-highlight/issues/1)
• [x] Add and check formal grammar (#11)
• [x] Check test suite
• [x] proofread homepage
• [x] add authors to specification and remove note about work in progress
• [x] publish with DOI (Zenodo), at arXiv or via another formal publication (academic paper?)

We may use another version number such as 0.5.0 as long as every following changes are fully-backwards-compatible (PG format documents must not become invalid in updated versions).