docs: add docs that detail the asset chan funding process

[Roasbeef]

In this commit, we add some additional docs that explain how asset channels differ from normal channels, and some of the interaction that happens with lnd using the set of relevant Aux* interfaces that tapd implements.

[GeorgeTsagk]

Lgtm

All the non-sequence diagrams felt a bit vague / unnecessary, but since this is a high-level doc that only aims to outline what's happening we might as well leave them as is.

[coveralls]

Pull Request Test Coverage Report for Build 18145671859

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 62 unchanged lines in 14 files lost coverage.
• Overall coverage decreased (-0.01%) to 56.52%

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Files with Coverage Reduction	New Missed Lines	%
asset/asset.go	2	79.97%
mssmt/compacted_tree.go	2	78.11%
rfqmsg/records.go	2	70.8%
tapchannel/aux_leaf_signer.go	2	43.18%
tapdb/sqlc/assets.sql.go	2	69.03%
tapdb/universe.go	2	80.58%
tapgarden/custodian.go	2	76.83%
universe/supplyverifier/manager.go	2	64.53%
asset/mock.go	3	73.21%
tapgarden/caretaker.go	4	76.63%

Totals
Change from base Build 18145443127:	-0.01%
Covered Lines:	63788
Relevant Lines:	112859

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💛 - Coveralls