Rename filenames to all lowercase for consistency and cleaner URLs

[bact]

For cleaner URLs, propose to rename these files:

• SPDX-Lite.md to spdx-lite.md
• SPDX-license-expressions.md to spdx-license-expressions.md

Other files are already in all-lowercase, including ones with a word which originally an all-uppercase initialism:

• changes-from-previous-iso.md (ISO)
• pkg-url-specification.md (URL)
• rdf-model.md (RDF)

The all-lowercase spdx will also match the other spdx instances in the full URL of: https://spdx.github.io/spdx-spec/...

I'm suggesting we have this before the publication of v3.0.1 website, so the URLs stay the same.