docs: make redocly config explicit (#1650)

Per discussion on #1633, it looks like `redocly` config needs to be more explicit.

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:

```sh
$ 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
```

Author: alxndrsn

Makefile

@@ -168,4 +168,4 @@ api-docs:
 .PHONY: api-docs-lint
 api-docs-lint:
 	node lib/bin/openapi-docs-lint.js docs/api.yaml && \
-	npx --no @redocly/cli lint --extends=minimal docs/api.yaml
+	npx --no @redocly/cli lint --config docs/redocly.conf.yaml docs/api.yaml

docs/redocly.conf.yaml

@@ -0,0 +1,32 @@
+rules:
+  no-ambiguous-paths: warn
+  no-empty-servers: warn
+  no-enum-type-mismatch: warn
+  no-example-value-and-externalValue: warn
+  no-identical-paths: warn
+  no-invalid-media-type-examples:
+    severity: warn
+    allowAdditionalProperties: false
+  no-path-trailing-slash: warn
+  no-required-schema-properties-undefined: warn
+  no-schema-type-mismatch: warn
+  no-server-example.com: warn
+  no-server-trailing-slash: error
+  no-server-variables-empty-enum: error
+  no-undefined-server-variable: warn
+  no-unresolved-refs: error
+  no-unused-components: warn
+  nullable-type-sibling: warn
+  operation-2xx-response: warn
+  operation-operationId: warn
+  operation-operationId-unique: warn
+  operation-operationId-url-safe: warn
+  operation-parameters-unique: warn
+  operation-summary: warn
+  path-declaration-must-exist: warn
+  path-not-include-query: warn
+  path-parameters-defined: warn
+  security-defined: warn
+  spec-components-invalid-map-name: warn
+  struct: error
+  tag-description: warn