Add LVI attacks

[jovanbulck]

We should incorporate https://lviattack.eu/ into the tree. Either as an addition to the MD subtree or a separate branch? Given the symmetry with existing MD-type attacks, Id argue for extending the MD subtree instead of creating a new branch. Proposed plan RFC:

• [ ] rename Meltdown-type to Meltdown/LVI-type to reflect that MD is not anymore only about "melting down" protection domains by leaking
• [ ] update each MD leaf with a "leakage" and "injection" subsection briefly describing the impact of the leakage/injection angles, if applicable
• [ ] add NULL as an extra leaf for the last-level u-arch buffer split. At least for MD-US-NULL, we can also mention EchoLoad in the "leakage"-oriented subsection, as it abuses NULL forwarding to break ASLR which leaks side-channel info.
• [ ] add a new MD-PPN node to describe Foreshadow-EPCM SGX leakage attacks and LVI-PPN-L1D attacks; for completeness we should prob also add MD-RSVD

Ideas, feedback?