[ENH] Formalize presence of optional logs/ folder

[yarikoptic]

It is quite often desired to store logs, e.g. simply stdout/stderr from invocation of BIDS converters or other tools which were used to produce/change content in current BIDS datasset. They quite often provide ultimate provenance information to troubleshoot odd or incorrect results. But researchers, in my personal opinion, undervalue logs! But hinting them on "standard" location for them, I think we could inspire more of relevant to dataset provenance metadata being collected and shared.

TODOs:

• ~~?Danger: might leak sensitive metadata, so may be wording should be adjusted to mention that they should be inspected/sensored? WDYT?~~ I think it is fine/assumed

Shameless plug: consider using con-duct (https://github.com/con/duct) for your "logging needs".

Note that I think that ideally the logs/ should not include "derivative" data, such as e.g. output of bids-validator. Those outputs should go under derivatives/bids-validator-{version}/output.json or alike. (It might be the only reasonable "derivative" to recommend bundling with any "raw" or derived BIDS dataset).