docs: add docs on e2e RFQ arch

[Roasbeef]

Tried to make this one very comprehensive, but it can likely be split up into smaller docs.

[coveralls]

Pull Request Test Coverage Report for Build 18142751393

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.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• 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.
• 6275 unchanged lines in 71 files lost coverage.
• Overall coverage decreased (-0.5%) to 56.483%

---

Files with Coverage Reduction	New Missed Lines	%
commitment/tap.go	1	85.65%
fn/context_guard.go	1	91.94%
universe/supplyverifier/env.go	1	87.5%
asset/asset.go	2	80.3%
mssmt/node.go	2	98.76%
mssmt/proof.go	2	97.59%
tapdb/sqlc/transfers.sql.go	2	83.33%
tapgarden/custodian.go	2	77.02%
taprpc/taprootassets_grpc.pb.go	2	60.33%
universe_rpc_diff.go	2	76.0%

Totals
Change from base Build 17562325369:	-0.5%
Covered Lines:	63674
Relevant Lines:	112731

---

💛 - Coveralls

[jtobin]

I agree that this is very (too) long, but it can be split up in a separate effort. The content LGTM, though see my PR to your branch for minor stylistic suggestions.

[lightninglabs-deploy]

@ffranr: review reminder