slsa.dev 2.0 (PR 1/x): Content and structure updates

[devmoran]

• Adds subpages and content
• New docs/index.md
• Updates project README

To be followed by a PR for layout and styling updates

[MarkLodato]

Thanks, Dev! Sorry for so many comments - it's a big PR! :-D

[MarkLodato]

Regarding the link renaming, we're using markdownlint-cli instead of prettier since the former is less opinionated.

[inferno-chromium]

@devmoran - looks like linter is failing.
Can you please do that definition update (see comment above) and these minor lint fixes

docs/getinvolved.md:3 MD001/heading-increment/header-increment Heading levels should only increment by one level at a time [Expected: h2; Actual: h4]
docs/getinvolved.md:7 MD001/heading-increment/header-increment Heading levels should only increment by one level at a time [Expected: h3; Actual: h4]
docs/levels.md:3 MD001/heading-increment/header-increment Heading levels should only increment by one level at a time [Expected: h2; Actual: h4]
docs/levels.md:53 MD036/no-emphasis-as-heading/no-emphasis-as-header Emphasis used instead of a heading [Context: "○ = required unless there is a..."]
docs/requirements.md:3 MD001/heading-increment/header-increment Heading levels should only increment by one level at a time [Expected: h2; Actual: h4]
use_cases.md:80:1 MD005/list-indent Inconsistent indentation for list items at the same level [Expected: 4; Actual: 6]
use_cases.md:81:1 MD005/list-indent Inconsistent indentation for list items at the same level [Expected: 4; Actual: 6]
Error: Process completed with exit code 1.

[MarkLodato]

I pushed a few cleanup commits but otherwise LGTM. Thanks, Dev and company!

[inferno-chromium]

Thanks @devmoran and @MarkLodato !