[ci] Fix version matching between RTD pages and R-package pages (#6673)

* Update script.js

* Update script.js

* Update script.js

* Update script.js

* replace url at build time

* manipulate with raw files

docs/_static/js/script.js

@@ -4,25 +4,12 @@ $(function() {
 
     /* List each class property item on a new line
        https://github.com/microsoft/LightGBM/issues/5073 */
-    if(window.location.pathname.toLocaleLowerCase().indexOf('pythonapi') != -1) {
+    if(window.location.pathname.toLocaleLowerCase().indexOf('pythonapi') !== -1) {
         $('.py.property').each(function() { this.style.setProperty('display', 'inline', 'important'); });
     }
 
-    /* Point to the same version of R API as the current docs version */
-    var current_version_elems = $('.rst-current-version');
-    if(current_version_elems.length !== 0) {
-        var current_version = $(current_version_elems[0]).contents().filter(function() {
-            return this.nodeType == 3;
-        }).text().trim().split(' ').pop();
-        if(current_version !== 'latest') {
-            $('a.reference.external[href$="/latest/R/reference/"]').each(function() {
-                $(this).attr('href', function (_, val) { return val.replace('/latest/', '/' + current_version + '/'); });
-            });
-        }
-    }
-
     /* Collapse specified sections in the installation guide */
-    if(window.location.pathname.toLocaleLowerCase().indexOf('installation-guide') != -1) {
+    if(window.location.pathname.toLocaleLowerCase().indexOf('installation-guide') !== -1) {
         $('<style>.closed, .opened {cursor: pointer;} .closed:before, .opened:before {font-family: FontAwesome; display: inline-block; padding-right: 6px;} .closed:before {content: "\\f078";} .opened:before {content: "\\f077";}</style>').appendTo('body');
         var collapsable = [
             '#build-threadless-version-not-recommended',

docs/conf.py

@@ -39,6 +39,7 @@ LIB_PATH = CURR_PATH.parent / "python-package"
 sys.path.insert(0, str(LIB_PATH))
 
 INTERNAL_REF_REGEX = compile(r"(?P<url>\.\/.+)(?P<extension>\.rst)(?P<anchor>$|#)")
+RTD_R_REF_REGEX = compile(r"(?P<begin>https://.+/)(?P<rtd_version>latest)(?P<end>/R/reference/)")
 
 
 class InternalRefTransform(Transform):
@@ -69,6 +70,7 @@ class IgnoredDirective(Directive):
 os.environ["LIGHTGBM_BUILD_DOC"] = "1"
 C_API = os.environ.get("C_API", "").lower().strip() != "no"
 RTD = bool(os.environ.get("READTHEDOCS", ""))
+RTD_VERSION = os.environ.get("READTHEDOCS_VERSION", "stable")
 
 # If your documentation needs a minimal Sphinx version, state it here.
 needs_sphinx = "2.1.0"  # Due to sphinx.ext.napoleon, autodoc_typehints
@@ -309,6 +311,22 @@ def generate_r_docs(app: Sphinx) -> None:
         raise Exception(f"An error has occurred while generating documentation for R-package\n{e}")
 
 
+def replace_reference_to_r_docs(app: Sphinx) -> None:
+    """Make reference to R-package documentation point to the actual version.
+
+    Parameters
+    ----------
+    app : sphinx.application.Sphinx
+        The application object representing the Sphinx process.
+    """
+    index_doc_path = CURR_PATH / "index.rst"
+    with open(index_doc_path, "r+t", encoding="utf-8") as index_doc:
+        content = index_doc.read()
+        content = RTD_R_REF_REGEX.sub(rf"\g<begin>{RTD_VERSION}\g<end>", content)
+        index_doc.seek(0)
+        index_doc.write(content)
+
+
 def setup(app: Sphinx) -> None:
     """Add new elements at Sphinx initialization time.
 
@@ -330,6 +348,7 @@ def setup(app: Sphinx) -> None:
         app.connect(
             "build-finished", lambda app, _: copytree(CURR_PATH.parent / "lightgbm_r" / "docs", Path(app.outdir) / "R")
         )
+    app.connect("builder-inited", replace_reference_to_r_docs)
     app.add_transform(InternalRefTransform)
     add_js_file = getattr(app, "add_js_file", False) or app.add_javascript
     add_js_file("js/script.js")