Document community register aliases and recently identified registers

[Rangi42]

Fixes #605 by documenting the community aliases for these registers:

• KEY0 has the alias SYS. It was also missing from the "Hardware Reg List" and the "Power-Up Sequence" list.
• KEY1 has the alias SPD.
• HDMAx have the aliases VDMA_*. These are not fully listed, but just linked to, same as the NRxy aliases AUD*.
• SVBK has the alias WBK. (The alias SMBK is deprecated in hardware.inc 5.0.)

And some other discrepancies noted while making those edits:

• $FF50 has the name BANK. It was missing from the "Power-Up Sequence" list.
• PCM12, PCM34, and $FF72/3/4/5 had leftover link anchors in the "Power-Up Sequence" list. (The anchors for PCM12 and PCM34 led to undefined headings.)

[Rangi42]

CI is failing because "linkcheck" gets 403 Forbidden from three external links, all of which are still valid. 🤷‍♀️

[pinobatch]

I'll ask the forums.nesdev.org server staff what is going on. It might be related to a mitigation for a traffic flood a few weeks ago.

[Rangi42]

Pan Docs current master also fails linkcheck, so it's at least not a blocker for this PR.