[pydocstyle] Skip leading whitespace for D403 (#14963)

This PR introduces three changes to `D403`, which has to do with
capitalizing the first word in a docstring.

1. The diagnostic and fix now skip leading whitespace when determining
what counts as "the first word".
2. The name has been changed to `first-word-uncapitalized` from
`first-line-capitalized`, for both clarity and compliance with our rule
naming policy.
3. The diagnostic message and documentation has been modified slightly
to reflect this.

Closes #14890

crates/ruff_linter/resources/test/fixtures/pydocstyle/D403.py

@@ -31,3 +31,13 @@ def single_word():
 
 def single_word_no_dot():
     """singleword"""
+
+def first_word_lots_of_whitespace():
+    """
+
+
+
+    here is the start of my docstring!
+
+    What do you think?    
+    """