Added fetchPrice example, reorganized HC docs

[katieclay]

:clipboard: Add associated issues, tickets, docs URL here.

Overview

Added a brief explanation of the fetchPrice contract

Changes

Describe your changes and implementation choices. More details make PRs easier to review.

• Added new file for fetchPrice
• Reorganized HC docs to accommodate for both examples, including TOC tree

Testing

n/a

[katieclay]

Need to ask about server.register_function(offchain_addsub2, selector("offchain_addsub2(uint32,uint32)")); other example selector()s' parameter functions don't have "offchain_" in them.

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.10%. Comparing base (7d1a7c2) to head (91b7a5a).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #247 +/- ## ======================================== Coverage 78.10% 78.10% ======================================== Files 28 28 Lines 2137 2137 ======================================== Hits 1669 1669 Misses 407 407 Partials 61 61 ``` | [Flag](https://app.codecov.io/gh/bobanetwork/boba/pull/247/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bobanetwork) | Coverage Δ | | |---|---|---| | [cannon-go-tests](https://app.codecov.io/gh/bobanetwork/boba/pull/247/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bobanetwork) | `78.10% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bobanetwork#carryforward-flags-in-the-pull-request-comment) to find out more.