git: openjdk/amber-docs: Update publishing infra; update index to match existing project page; swap md -> html in links

[duke]

Changeset: 8de720a8
Author:    briangoetz <brian at briangoetz.com>
Date:      2021-05-13 16:42:19 +0000
URL:       https://git.openjdk.java.net/amber-docs/commit/8de720a8174f1af86351c27aeb988cfe2c5a9e94

Update publishing infra; update index to match existing project page; swap md -> html in links

+ .github/workflows/publish.yaml
! .gitignore
! Makefile
+ etc/footer.html
= etc/style.css
! site/design-notes/extending-switch-for-patterns.md
! site/design-notes/pattern-match-object-model.md
! site/design-notes/pattern-match-semantics.md
! site/design-notes/pattern-matching-for-java.md
! site/design-notes/type-patterns-in-switch.md
! site/index.md