[`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
