chore: Unified Sidebar menu collection using a sidebar loader

[YaoYinYing]

I think static sidebar in every html looks good if user opens the file directly to web explorers. Yet this not only introduces tons of repetative code, but also requires heavy workload on maintainance work like updating since every html needs to be updated if new functionly html needs to be added or removed.

Instead, reading menu code from a singlenav.html to form a dynamic sidebar could be more efficient. Also this may be more convenient for people who want to get this project translated (i18n issue).

Example pages just work fine:

1. (mirrored from my fork using Cloudflare Pages) https://biosequence.yaoyy.moe/
2. (mirrored for GitHub Pages test): https://yaoyinying.github.io/sequence_manipulation_suite

Now here is the issue:
when user directly open any of the htmls from their double clicks, menu wont be shown due to mismatch of nav.html relative path.

To solve this, one must use python3 -m http.server 8080 under docs and open http://localhost:8080 on their browser as a local website.