Document feature spec readiness waits

What does this MR do and why?

Documents reliable readiness and synchronization patterns for feature specs.

• Replaces the accessibility-guide example that recommends wait_for_requests.
• Adds canonical guidance on why wait_for_requests and wait_for_all_requests race, and links the RuboCop cop directly to it.
• Documents concrete replacements from !246008 (merged): waiting matchers, modal readiness, wait_for polling, script reads, shared-example gates, and narrowly scoped JavaScript readiness signals.
• Clarifies that have_gitlab_http_status accepts a Capybara session.

Related issues

Resolves #608154

References

• !246008 (merged)
• &22879
• Slack thread: https://gitlab.slack.com/archives/C9X79MNJ3/p1785916768087989?thread_ts=1785894398.639519&cid=C9X79MNJ3

How to set up and validate locally

1. Run bundle exec rspec spec/rubocop/cop/rspec/avoid_wait_for_requests_spec.rb.
2. Run bundle exec rubocop rubocop/cop/rspec/avoid_wait_for_requests.rb spec/rubocop/cop/rspec/avoid_wait_for_requests_spec.rb.
3. Run scripts/lint-doc.sh doc/development/fe_guide/accessibility/feature_tests.md doc/development/fe_guide/style/javascript.md doc/development/testing_guide/best_practices.md doc/development/testing_guide/unhealthy_tests.md.
4. Run lychee --offline --include-fragments doc/development/fe_guide/accessibility/feature_tests.md doc/development/fe_guide/style/javascript.md doc/development/testing_guide/best_practices.md doc/development/testing_guide/unhealthy_tests.md.

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