docs: fix links & headings about built-in samplers

[pokutuna]

Which problem is this PR solving?

• Some of links about samplers are dead in packages/opentelemetry-core/README.md.

Short description of the changes

• Make them links to existing headings.
• Change the heading text to add Sampler like about Propagator section.

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ pokutuna (d334285)

[codecov]

Codecov Report

Merging #2011 (fd6b86c) into main (0c7fef7) will increase coverage by 3.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2011      +/-   ##
==========================================
+ Coverage   92.97%   96.04%   +3.06%     
==========================================
  Files         152       12     -140     
  Lines        5925      632    -5293     
  Branches     1245      126    -1119     
==========================================
- Hits         5509      607    -4902     
+ Misses        416       25     -391     

Impacted Files	Coverage Δ
...ckages/opentelemetry-exporter-zipkin/src/zipkin.ts	81.63% <0.00%> (-18.37%)	⬇️
...strumentation/src/platform/node/instrumentation.ts
packages/opentelemetry-propagator-b3/src/common.ts
...ntelemetry-propagator-b3/src/B3SinglePropagator.ts
...telemetry-core/src/common/logging-error-handler.ts
packages/opentelemetry-sdk-node/src/sdk.ts
...ackages/opentelemetry-metrics/src/CounterMetric.ts
...s/opentelemetry-plugin-grpc-js/src/client/index.ts
packages/opentelemetry-plugin-grpc/src/utils.ts
...es/opentelemetry-propagator-b3/src/B3Propagator.ts
... and 131 more