docs: use SPDX-standard Apache 2.0 LICENSE text

The previous LICENSE had a non-standard leading-whitespace format
from a copy-paste, which broke GitHub's license detection (showed
'Other' / 'NOASSERTION' in the repo sidebar).

Replaced with the canonical SPDX text (https://spdx.org/licenses/Apache-2.0.html)
plus the standard 'Copyright YEAR NAME' block.

GitHub's licensee gem will now correctly identify the file as
Apache-2.0 after the next commit (detection is async, can take
a few minutes to propagate).

Author: gilbertwong96