Add/fix documentation for usage of `open bugs` page under `test/functional/test_site/bugs`

[tlylt]

Is your request related to a problem?
Related to PR #1148 and issue #1140.

The original issue called for the removal of bug demo code found in test/functional/test_site/bugs/index.md. I believe these were used as a way to illustrate certain bugs (as seen in these two comments below).
(link here and here.) Screenshot for easy reference:

---

I am guessing that bugs/index.md is still in use (and useful) when reproducing bugs. However, there is no mention of such usage in our developer guide. Simply by browsing through the files, the content in bugs/index.md is not as clear to suggest to developers who happen to chance upon it that this is used to report bugs. I initially thought that it was some kind of template used when writing test cases, but I couldn't find any references.

<frontmatter>
title: Open Bugs
header: header.md
</frontmatter>

<div class="website-content">

**Bug Description**

<a href=LINK_TO_MARKBIND_ISSUE>Issue #XX</a>

Repro:

Steps to Reproduce

</div>

After some digging, I found out that in our issues template, there is a liner that refers to this:

<!-- You are encouraged to submit a PR that reproduces this in test/test_site/bugs/. -->

Describe the solution you'd like

I think the issue template is perhaps outdated or that the folder path is no longer accurate. If that is the case, we should

• Update path to the bug-reproducing page in the issue template

so that bug reporters can find out where to reproduce a bug.

Describe alternatives you've considered

Lower priority but perhaps we could also:

• Highlight how to reproduce bugs using test_site/bugs/ in the developer guide

for new contributors to note that they might want to make use of this when exploring the codebase or when creating issues related to certain bugs that they discover.
Or

• Include a paragraph of instructions/explainations in bugs/index.md as to how this page is used