README: reformat and reorganize

- change HTML syntax to markdown syntax
- move the (now rather lengthy) authors section below the (more
  important) API overview section
- merge (and reword) the contributors and authors sections, which were a
  bit redundant
- also remove the (now redundant) support section
10 jobs for v1.18.0 in 2 minutes and 22 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #258033
debian aarch64
build-debian-aarch64

00:00:32

passed #258034
macos
build-macos

00:00:04

passed #258029
amd64 debian
build-ubuntu

00:00:31

23.0%
passed #258030
amd64 debian
build-ubuntu-static

00:00:25

passed #258031
win32
build-win32

00:00:22

passed #258032
win64
build-win64

00:00:25

 
  Test
passed #258035
amd64 debian
test-ubuntu-asan

00:00:28

passed #258036
amd64 debian
test-ubuntu-msan

00:00:24

passed #258038
amd64 debian
test-ubuntu-scan

00:00:54

passed #258037
amd64 debian
test-ubuntu-ubsan

00:00:25