docs: add generated and reviewed developer deep-dive docs

[guggero]

This PR adds three documents that were initially generated by agentic LLMs but now reviewed by me.
I've read through all three documents (kudos to @Roasbeef and @bhandras for their prompts that generated very high-quality initial drafts) and edited them where necessary.

With the remaining time available, I aimed for 80 to 90% accuracy. Meaning the documents shouldn't be taken for the absolute truth (there is a disclaimer at the top) but should in any case be a valuable entry point into learning about tapd for any developer (with one document targeting users of tapd and its RPCs and the other document contributors to tapd's code itself).

[coveralls]

Pull Request Test Coverage Report for Build 17159270515

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 28 unchanged lines in 10 files lost coverage.
• Overall coverage increased (+0.004%) to 56.619%

---

Files with Coverage Reduction	New Missed Lines	%
address/mock.go	2	88.59%
mssmt/compacted_tree.go	2	77.97%
rfqmsg/records.go	2	71.29%
tapdb/assets_common.go	2	78.34%
tapdb/mssmt.go	2	89.55%
tapdb/sqlc/transfers.sql.go	2	82.65%
universe/archive.go	3	79.29%
tapdb/universe.go	4	81.37%
tapgarden/caretaker.go	4	76.93%
tapdb/assets_store.go	5	79.2%

Totals
Change from base Build 17140538774:	0.004%
Covered Lines:	60795
Relevant Lines:	107376

---

💛 - Coveralls

[lightninglabs-deploy]

@bhandras: review reminder
@jtobin: review reminder

[Roasbeef]

LGTM 🪉