feat: initial storage slots docs

[LHerskind]

Adds an initial explanation around storage slots, how they are computed and used between private and public data.

Checklist:

Remove the checklist to signal you've completed it. Enable auto-merge if the PR is ready to merge.

• If the pull request requires a cryptography review (e.g. cryptographic algorithm implementations) I have added the 'crypto' tag.
• I have reviewed my diff in github, line by line and removed unexpected formatting changes, testing logs, or commented-out code.
• Every change is related to the PR description.
• I have linked this pull request to relevant issues (if any exist).

[AztecBot]

Benchmark results

No metrics with a significant change found.

Detailed results

All benchmarks are run on txs on the Benchmarking contract on the repository. Each tx consists of a batch call to create_note and increment_balance, which guarantees that each tx has a private call, a nested private call, a public call, and a nested public call, as well as an emitted private note, an unencrypted log, and public storage read and write.

This benchmark source data is available in JSON format on S3 here.

Values are compared against data from master at commit d707d4e3 and shown if the difference exceeds 1%.

L2 block published to L1

Each column represents the number of txs on an L2 block published to L1.

Metric	8 txs	32 txs	128 txs
l1_rollup_calldata_size_in_bytes	45,444	179,588	716,132
l1_rollup_calldata_gas	222,948	868,052	3,449,504
l1_rollup_execution_gas	842,035	3,595,160	22,204,873
l2_block_processing_time_in_ms	1,977 (-1%)	7,591	29,555 (-1%)
note_successful_decrypting_time_in_ms	292 (-2%)	884	3,194 (-1%)
note_trial_decrypting_time_in_ms	94.0 (-6%)	91.0 (-6%)	135 (+1%)
l2_block_building_time_in_ms	13,159 (-1%)	52,553	209,577
l2_block_rollup_simulation_time_in_ms	11,816 (-1%)	47,228	188,385
l2_block_public_tx_process_time_in_ms	1,299	5,185 (-1%)	20,697 (-1%)

L2 chain processing

Each column represents the number of blocks on the L2 chain where each block has 16 txs.

Metric	5 blocks	10 blocks
node_history_sync_time_in_ms	21,232 (-3%)	41,696 (-2%)
note_history_successful_decrypting_time_in_ms	1,989 (-2%)	3,888 (-2%)
note_history_trial_decrypting_time_in_ms	120	143
node_database_size_in_bytes	1,631,363	1,101,473
pxe_database_size_in_bytes	29,748	59,307

Circuits stats

Stats on running time and I/O sizes collected for every circuit run across all benchmarks.

Circuit	circuit_simulation_time_in_ms	circuit_input_size_in_bytes	circuit_output_size_in_bytes
private-kernel-init	775	61,697	18,905
private-kernel-ordering	123	24,297	8,153
base-rollup	1,767	656,311	814
root-rollup	80.9 (+4%)	4,072	1,097
private-kernel-inner	789	81,568	18,905
public-kernel-private-input	41.3 (-1%)	41,519	18,841
public-kernel-non-first-iteration	26.3 (-1%)	41,497	18,841
merge-rollup	0.971 (+5%)	2,592	873

Miscellaneous

Transaction sizes based on how many contracts are deployed in the tx.

Metric	0 deployed contracts	1 deployed contracts
tx_size_in_bytes	8,787	27,547

[benesjan]

Brilliant writeup 👍 Just a few comments and suggestions.

[iAmMichaelConnor]

(there will be more)

What's this alluding to?

[LHerskind]

More siloing, see line 56.

[rahul-kothari]

change there "more on this later" then

[iAmMichaelConnor]

I had a quick skim through when I saw Jan had approved it. (I was delaying looking at this).
Just some minor grammatical changes, really.
Hopefully this will give some much-needed intuition to devs; thanks for writing this in such a well-structured way.

We might need sign-off from devrel before merging this (new policy for docs).

[rahul-kothari]

s/leafs/leaves

Slow to check it again.