fix(docs): replace stale v3 doc links with v4 equivalents (#2786)

Customers following doc links from the main branch were landing on v3
(SodaCL / Soda Library) documentation and following the wrong process.

- JSON contract schema: point the schema check description at the v4
  contract language reference (#schema-check) instead of the v3 SodaCL
  page, and the threshold descriptions at #thresholds instead of the
  dead docs.soda.io/#thresholds anchor. Also drop two leftover 'YY '
  prefixes in threshold descriptions.
- exit_codes.py: the ExitCode docstring linked to the v3 Soda Library
  docs; no v4 page documents Soda Core exit codes (the sodacli page
  documents different semantics), so document the codes inline instead.

The v3 links in the README's 'Working with legacy Soda Core v3' section
are intentional and left as-is.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: mivds

soda-core/src/soda_core/cli/exit_codes.py

@@ -5,7 +5,11 @@
 class ExitCode(IntEnum):
     """Possible exit codes and their meaning.
 
-    See https://docs.soda.io/soda-library/programmatic.html#scan-exit-codes
+    0: all checks passed, no errors
+    1: one or more checks failed
+    2: one or more checks warned (no failures)
+    3: errors occurred during execution (e.g. parse or engine errors)
+    4: results could not be sent to Soda Cloud
     """
 
     OK = 0

soda-core/src/soda_core/contracts/soda_data_contract_json_schema_1_0_0.json

@@ -680,7 +680,7 @@
                                     {
                                         "type": "object",
                                         "additionalProperties": false,
-                                        "description": "Use a schema check to validate the presence, absence or position of columns in a dataset, or to validate the type of data column contains. See documentation for more details: https://docs.soda.io/soda-cl/schema.html",
+                                        "description": "Use a schema check to validate the presence, absence or position of columns in a dataset, or to validate the type of data column contains. See documentation for more details: https://docs.soda.io/soda-v4/reference/contract-language-reference#schema-check",
                                         "properties": {
                                             "name": {
                                                 "description": "The display name for the check used in Soda Cloud",
@@ -1121,7 +1121,7 @@
         "threshold": {
             "type": "object",
             "additionalProperties": false,
-            "description": "The threshold for the check. See documentation for more details: https://docs.soda.io/#thresholds",
+            "description": "The threshold for the check. See documentation for more details: https://docs.soda.io/soda-v4/reference/contract-language-reference#thresholds",
             "properties": {
                 "metric": {
                     "description": "The type of the metric value to check",
@@ -1131,7 +1131,7 @@
                     ]
                 },
                 "must_be": {
-                    "description": "YY The value the check metric (as specified in the type) must have for the check to pass; The check passes if the metric has the specified value, and fails otherwise; https://docs.soda.io/#thresholds",
+                    "description": "The value the check metric (as specified in the type) must have for the check to pass; The check passes if the metric has the specified value, and fails otherwise; https://docs.soda.io/soda-v4/reference/contract-language-reference#thresholds",
                     "type": [
                         "number",
                         "string"
@@ -1296,7 +1296,7 @@
         "thresholdFreshness": {
             "type": "object",
             "additionalProperties": false,
-            "description": "The threshold for the check. See documentation for more details: https://docs.soda.io/#thresholds",
+            "description": "The threshold for the check. See documentation for more details: https://docs.soda.io/soda-v4/reference/contract-language-reference#thresholds",
             "properties": {
                 "unit": {
                     "description": "The type of the metric value to check",
@@ -1307,7 +1307,7 @@
                     ]
                 },
                 "must_be": {
-                    "description": "YY The value the check metric (as specified in the type) must have for the check to pass; The check passes if the metric has the specified value, and fails otherwise; https://docs.soda.io/#thresholds",
+                    "description": "The value the check metric (as specified in the type) must have for the check to pass; The check passes if the metric has the specified value, and fails otherwise; https://docs.soda.io/soda-v4/reference/contract-language-reference#thresholds",
                     "type": "number"
                 },
                 "must_not_be": {