README.md: fix broken readthedocs link

The nice restructuring had these side effects, unfortunately.
26 jobs for broken-readthedocs-link in 11 minutes and 31 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Build
passed #361303
amd64 linux docker
archive

00:00:54

passed #361304
amd64 linux docker
build

00:00:58

passed #361305
amd64 linux docker
build-asan

00:00:54

passed #361306
docker
kres-gen

00:00:56

 
  Test
passed #361307
docker delayed
build:darwin

00:03:27

passed #361328
docker linux amd64
deckard

00:02:14

passed #361309
docker delayed
doc

00:03:40

passed #361310
dind
docker

00:02:19

passed #361311
docker delayed
lint:other

00:03:28

passed #361312
amd64 linux docker delayed
lint:pedantic

00:03:29

passed #361313
amd64 linux docker
lint:scan-build

00:02:38

passed #361314
amd64 linux docker
lint:tidy

00:03:35

passed #361315
amd64 linux docker
respdiff:basic

00:03:06

passed #361316
amd64 linux docker delayed
test:valgrind

00:04:28

failed #361308
amd64 linux docker
deckard

00:03:07

 
  Extended
passed #361318
respdiff
fwd-tls6-kresd.udp6

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passed #361319
respdiff
fwd-udp6-kresd.udp6

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passed #361323
respdiff
fwd-udp6-unbound.tcp6

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passed #361324
respdiff
fwd-udp6-unbound.tls6

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passed #361322
respdiff
fwd-udp6-unbound.udp6

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passed #361321
respdiff
iter.tls6

00:00:17

passed #361320
respdiff
iter.udp6

00:00:18

passed #361317
amd64 linux docker
pytests

00:04:00

passed #361325
respdiff
resperf:fwd-tls6.udp-asan

00:00:17

passed #361326
respdiff
resperf:fwd-udp6.udp-asan

00:00:17

passed #361327
respdiff
resperf:iter.udp-asan

00:00:17