style: optimize heading/ structure on website

src/collation.md

src/de/impressum.md

src/de/index.md

src/de/literatur.md

src/de/motive.md

src/de/richtlinenedition.md

src/de/stemmata.md

src/edition.md

src/editionguidelines.md

src/imprint.md

src/index.md

src/manuscripts.md

src/motifs.md

src/privacy.md

src/project.md

src/relatedliterature.md

src/search.md

src/stemmata.md

src/termsofuse.md