doc: add generation of tags from sdkconfig and x_caps.h headers

Also updates the way we handle exclude_patterns to use the new tags

Closes IDF-1484

docs/idf_extensions/exclude_docs.py

@@ -0,0 +1,12 @@
+# Updates the excluded documents according to the conditional_include_dict {tag:documents}
+def update_exclude_patterns(app, config):
+    for tag, docs in config.conditional_include_dict.items():
+        if not app.tags.has(tag):
+            app.config.exclude_patterns.extend(docs)
+
+
+def setup(app):
+    # Tags are generated together with defines
+    app.connect('config-inited', update_exclude_patterns)
+
+    return {'parallel_read_safe': True, 'parallel_write_safe': True, 'version': '0.1'}