docs: author per-target hardware descriptions for explanations/hardware-targets

[coretl]

Target page

explanations/hardware-targets

The page is a full stub — no content has been written yet. It should describe
each supported hardware target so integrators know which platform to choose.

Category: author-from-scratch

What a human must do (~4 hours)

For each MACHINE value in reference/machine-targets (pandabox, pandabox2,
pandabrick, plus the development-only xu5-st1 and zedboard — the list in
conf/machine/), provide:

1. **SoC / module** — the Xilinx SoC or module part number.
2. **Key I/O** — number and type of I/O connectors, FMC slots, SFP, etc.
3. **Distinguishing features** — what makes this target different from the
   others (performance, I/O count, form factor).
4. **Typical use case** — e.g. "general purpose lab PandA", "high-channel-count
   detector trigger".
5. **Links** — hardware datasheet or purchase page where publicly available.

A table-per-target or a definition-list format is fine. Post as a comment or
PR against explanations/hardware-targets.md.

Source

Interview6 §A (background); cross-reference reference/machine-targets.md.