docs: Adding basic values for capacitance + resistance.

Values taken from the following Google Spreadsheet
[SKY130 Stackup Capacitance Data" spreadsheet](https://docs.google.com/spreadsheets/d/1N9To-xTiA7FLfQ1SNzWKe-wMckFEXVE9WPkPPjYkaxE/edit#gid=226894802)

Signed-off-by: Tim 'mithro' Ansell <me@mith.ro>
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