Documentation improvements

This is a placeholder for relatively small and low-priority CLI documentation issues. Will keep it open and update as I find more.

• improve split documentation so it clarifies the intended usage (https://github.com/Redocly/redocly-cli/issues/1822)
• explain how to use openapi-core features in JS - https://github.com/Redocly/redocly-cli/blob/main/packages/core/README.md
• add explanation about overriding severity of configurable rules
• improve eject and other realm-related commands docs (see https://github.com/Redocly/redocly-cli/issues/1847#issuecomment-2602717316)
• clarify the filter-in decorator behaviour (https://github.com/Redocly/redocly-cli/issues/1741)
• clarify the merging stragety of root and apis-level config (https://github.com/Redocly/redocly-cli/issues/1871#issuecomment-2634020349)
• clarify that join doesn't run decorators (https://github.com/Redocly/redocly-cli/issues/1889#issuecomment-2642579978) since https://github.com/Redocly/redocly-cli/pull/1046 and https://github.com/Redocly/redocly-cli/pull/1507
• clarify that join doesn't modify the original descriptions (see https://github.com/Redocly/redocly-cli/issues/2263#issuecomment-3190921892)
• Review our openapi-startrer docs (see this comment: https://github.com/Redocly/redocly-cli/issues/2348#issuecomment-3435126140)
• Add explanation of config reusing stragegies (https://github.com/Redocly/redocly-cli/issues/2504#issuecomment-3852246649)
• TBD

cc @JLekawa

Start with the unchecked linked issues, especially 1822, 1847, 1741, 2348, and 2504, then locate the relevant CLI documentation and review the existing guidance. Done means each selected documentation gap is clarified and the corresponding checklist item can be marked complete.