docs: add Reference docs structure and add first signal proposal

[jaybxyz]

Description

closes: #147

Tasks

• Add Reference docs structure
• Add first signal proposal written by @dlguddus

References

• Reference docs in cosmos-cash project

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Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Appropriate labels applied
• Targeted PR against correct branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Re-reviewed Files changed in the Github PR explorer
• Review Codecov Report in the comment section below once CI passes

[Hyung-bharvest]

LGTM