docs: Add docs for PKCS#11 provider
The PKCS#11 provider is being released shortly.
This moves over the preliminary docs and cleans them up.
I added them as a new section under "Vault Enterprise" ->
"PKCS#11 Provider", but I am open to suggestion on if there is a
better place for them, e.g., "Platforms", or somehow merging
with "Vault Enterprise" -> "HSM"?
Co-authored-by: Theron Voran <tvoran@users.noreply.github.com>
Co-authored-by: Loann Le <84412881+taoism4504@users.noreply.github.com>
