Add v4.3 + geojson/csv add-on blog posts and docs

[a6b8]

Memo 094 P4a/P4b. Three new blog posts (EN+DE): v4.3 two-track release, GeoJSON add-on, CSV/TSV add-on. Generalise the about-page add-on/version table to the data-format add-on family + v4.3. Astro build green (179 pages, 6 new blog pages).