docs: make redocly config explicit

[alxndrsn]

Per discussion on #1633, it looks like redocly config will need to be updated for better compatibility in the future.

This PR extract the current minimal redocly config from source (https://github.com/Redocly/redocly-cli/blob/2367722a9fb173e55626dccbdac7b6694dd8b199/packages/core/src/config/minimal.ts#L52-L112) into a config file.

This should simplify tracking of future changes to this config.

Similarity to existing config can be confirmed with:

$ diff <(npx --no @redocly/cli lint --extends=minimal docs/api.yaml 2>&1) <(npx --no @redocly/cli lint --config docs/redocly.conf.yaml docs/api.yaml 2>&1)
1,2d0
< No configurations were provided -- using built in recommended configuration by default.
<
1447c1445
< docs/api.yaml: validated in 451ms
---
> docs/api.yaml: validated in 415ms

What has been done to verify that this works as intended?

diff <(npx --no @redocly/cli lint --extends=minimal docs/api.yaml 2>&1) <(npx --no @redocly/cli lint --config docs/redocly.conf.yaml docs/api.yaml 2>&1)

Why is this the best possible solution? Were any other approaches considered?

There isn't an alternative.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

Should help improve future support for parsing docs/api.yaml for users' OpenAPI clients.

Does this change require updates to the API documentation? If so, please update docs/api.yaml as part of this PR.

No.

Before submitting this PR, please make sure you have:

• run make test and confirmed all checks still pass OR confirm CircleCI build passes
• verified that any code from external sources are properly credited in comments or that everything is internally sourced