Document custom error handlers

[RedZapdos123]

Description:

Issue #1715 reported that OpenTelemetry Ruby supports custom error handlers, but the usage was not documented in an easy to find place.

This PR documents custom error handler configuration in the root README and adds YARD documentation for both OpenTelemetry.error_handler= and OpenTelemetry::SDK::Configurator#error_handler=.

It also adds a focused SDK regression test that verifies config.error_handler= is installed during SDK configuration.

Closes #1715.

Validations Run:

• docker compose run --rm api bundle exec rake test
• docker compose run --rm sdk bundle exec rake test
• docker compose run --rm api bundle exec rubocop lib/opentelemetry.rb
• docker compose run --rm sdk bundle exec rubocop lib/opentelemetry/sdk/configurator.rb test/opentelemetry/sdk_test.rb

Notes:

• The focused SDK RuboCop command reports 5 pre-existing Style/DocumentationMethod informational offenses in sdk/lib/opentelemetry/sdk/configurator.rb.
• bundle exec rake yard in both api and sdk still reports pre-existing repository YARD warnings unrelated to this change.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: RedZapdos123 / name: Mridankan Mandal (3f81159)

[RedZapdos123]

Following up on the notes in this PR description:

1. The pre-existing Style/DocumentationMethod offenses in sdk/lib/opentelemetry/sdk/configurator.rb are being handled in chore: add configurator method docs #2265.

2. The YARD @param tag cleanup was done in 2 scoped PRs: chore(api): clean up YARD param tags #2266 for the API and chore(sdk): clean up YARD param tags #2267 for the SDK.