Lowercase filenames for cleaner URLs

[bact]

Note - spdx/spdx-3-model#840 has to be applied first, before this PR.

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. (This is not affect the PDF generation)

Will fix #1051 (follow up of #1044 (comment) )

[bact]

These two warnings are to be fixed by spdx/spdx-3-model#840 in spdx-3-model

WARNING - Doc file 'model/ExpandedLicensing/ExpandedLicensing.md' contains a link '../../annexes/SPDX-license-expressions.md', but the target 'annexes/SPDX-license-expressions.md' is not found among documentation files.

WARNING - Doc file 'model/ExpandedLicensing/Classes/ConjunctiveLicenseSet.md' contains a link '../../../annexes/SPDX-license-expressions.md', but the target 'annexes/SPDX-license-expressions.md' is not found among documentation files.

[goneall]

LGTM

[bobmartin3000]

LGTM

[kestewart]

Looks good as soon as it's passing the checks.

[bact]

@kestewart Warnings fixed in spdx/spdx-3-model#840 -- have to merge that first in spdx-3-model, before this PR can pass the validation.

[kestewart]

Have merged spdx/spdx-3-model#840, so trying to get this to sort.

[kestewart]

@goneall - can you see if you can get the Validate build working.

[bact]

@kestewart it's validated now

[goneall]

spdx/spdx-3-model#840 was merged, so this can / must be merged for validation to pass