Simplify and update the project’s documentation #1

Could you tidy up the Markdown files? The README repeats “official” and “officially” 16 times, and the progress tables overlap quite a bit. The same status information is also repeated across several files.

Some paths need updating too: functions.po, stdtypes.po, and exceptions.po have moved from library/ to builtins/, but the tables, examples, and directory listings still use the old layout.

Please read through the docs and simplify them in your own words. Keeping the progress information in one place and linking to it would help. CONTRIBUTING.md and GLOSSARY.md could then focus on contribution instructions and terminology.