New symbols with updated names

[diadatp]

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Starts fixing #158.

A few minor edits to the device details page:

• Split the MIM cap symbol into m3 and m4 variants.
• Changed varactor cell name.
• Created a placeholder for the 20V NMOS zero-VT FET symbol.

The old_symbols page was created to help with comparison between the new and old symbols. It will be removed after PR review and commits are squashed.

The symbols have been created from commit c0191f6 of the Xschem SKY130 component library.

The VPP caps, diodes, ESD FETs and NPN BJTs symbols are WIP as some of them need further clarification/general docs cleanup and others need to be created.

[diadatp]

Comparison page is available here: https://skywater-pdk--284.org.readthedocs.build/en/284/old_symbols.html