Apply the closest redocly.yaml config to API docs

Is your feature request related to a problem? Please describe.

Currently, the config from the folder where the Redocly CLI is executed is used. This makes configuration more difficult, as users must either maintain separate config files for different API definitions and use the --config option (so they won't be applied automatically), or configure the apis section to apply rules or decorators differently for different files.

Describe the solution you'd like

A potential solution is to use the nearest redocly.yaml config when running a command like lint or bundle on a specific API file.

TBD: this changes how we treat config itself, and potentially the lint-ignore file.
Main questions:

• can we drop the apis section (since there will be no longer the 'main' config file)? (potentially, we can read and apply apis only when explicitly setting --config)
• performance issues
• how to save the lint-ignore files?
• how would it work for the join command? do we need a common config for this kind of commands?
• how to resolve config with the --config option set? should it simply override other config files?

Describe alternatives you've considered

Using current resolution method. The VS Code extension resolution method could be updated independently and Realm already resolves config independently.

Additional context

Originally came from the VS Code extension.

This could also be aligned with how Realm resolves config (see this comment).

As a part of this issue, we can stop supporting any default config file names other than redocly.yaml.

Start by tracing how the lint and bundle commands currently resolve configuration, then compare the referenced VS Code extension and Realm resolution approaches. Clarify nearest redocly.yaml behavior, --config precedence, join handling, and lint-ignore storage before defining completion criteria.