[Discover] Add context awareness design principles doc

[davismcphee]

Summary

Adds PRINCIPLES.md to Discover's context awareness framework, documenting the design principles a new profile should uphold (e.g. the ES|QL editor is always present, the data table always reflects the query 1:1, and interactions changing the table's results must be reflected in the query). README.md and DEV_DOCS.md are updated with links to the new doc.

Checklist

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• This was checked for breaking HTTP API changes, and any breaking changes have been approved by the breaking-change committee. The release_note:breaking label should be applied in these situations.
• Flaky Test Runner was used on any tests changed
• The PR description includes the appropriate Release Notes section, and the correct release_note:* label is applied per the guidelines
• Review the backport guidelines and apply applicable backport:* labels.

[infra-vault-gh-plugin-prod]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)

[kibanamachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 49fc361
• Build duration: 85 mins

Failed CI Steps

• Jest Tests #5
• Scout Lane #1 - stateful-classic / default
• Scout Lane #12 - stateful-classic / default

Test Failures

• [job] [logs] Jest Tests #5 / BackLink renders back to selection link when returnAppId and returnPath are present
• [job] [logs] Scout Lane #12 - stateful-classic / default / local-stateful-classic - Core Web Vitals - displays core web vitals metrics
• [job] [logs] Scout Lane #1 - stateful-classic / default / local-stateful-classic - Discover tabs - sharing - can share one unsaved tab from a persisted session

Metrics [docs]

✅ unchanged

cc @davismcphee