Increase visibility to the Symbol Technical Reference Guide

[orisdorch]

Request to prominently feature a link to the Technical Reference Guide on docs.symbol.dev site.

Proposed Technical Reference source URL: https://symbol.github.io/symbol-technicalref/main.pdf

It seems the existing documentation site only refers to the Technical Reference Document on a single nested page: https://docs.symbol.dev/server.html - considering the amount of information included in the Technical Reference, it seems sensible to feature it more prominently on the docs site.

I would recommend featuring it in the Technology section, separately introduced either before or after the list of Concept pages: https://docs.symbol.dev/concepts/overview.html [source file: https://github.com/symbol/symbol-docs/blob/main/source/concepts/overview.rst ].

Note -this may fall under the redesign (#527), though considering the relative simplicity of the change, it seemed to merit its own issue.