content: Add attested build environments level requirements

[marcelamelara]

This (draft) PR introduces the following spec changes associated with #975. Per https://github.com/slsa-framework/slsa/issues/975#issuecomment-1773315232 and https://github.com/slsa-framework/slsa/issues/975#issuecomment-1852559183 the spec enhancements are being proposed as a new Build track level. The spec changes introduced in this PR are meant to be complementary to possible requirements being developed in parallel in #977 .

Spec changes:

• [x] Add new level to levels.md
• [x] Add new requirements to requirements.md
• [x] Add related terminology to terminology.md
• [ ] Add example of hardware-based build environment attestation

Part 1 of #975 CC @chkimes

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[marcelamelara]

@mdwood-intel @pdxjohnny PTAL