Cross-reference docs/features.md from ADR 0034

[nkraetzschmar]

Update docs/features.md to cross-reference ADR 0034 as the canonical terminology reference for Garden Linux features.

• Add a link to ADR 0034 from docs/features.md.
• Point readers to ADR 0034 for the normative definitions instead of duplicating them further in builder documentation.

[yeoldegrove]

ADR 0034 and 0035 are linked inside this PR:

• https://github.com/gardenlinux/builder/pull/153/changes#diff--22cb7c154f490db40ba0fbc038297267af72615be95affe604bba96b51be53dR47
• https://github.com/gardenlinux/builder/pull/153/changes#diff-fbe630b89151395b5a334a0c59f2cacd86562874e153f512d2115509d83b4c9bR66