Modernize the SSL documentation for creating certificates

[esabol]

This PR updates the SSL documentation for creating CA, server, and client/worker certificates to modern usage with support for Subject Alternative Names (SAN).

[p-alik]

Docs-only change, low risk, technically sound. The switch to genpkey, -CAcreateserial, and especially the SAN extension block for the server cert are solid improvements — SAN support is genuinely needed since modern TLS clients reject CN-only hostname verification.

A few optional polish items for a follow-up, none blocking:

• The -nodes flag on the CA req -x509 -new -nodes -key gearmand-ca.key ... command is a no-op since -key supplies an already-unencrypted existing key (-nodes only matters when req generates a new key itself).
• Could mention that -CAcreateserial will auto-create gearmand-ca.srl on first use, since the doc no longer explains where the serial file comes from.
• Client/worker cert intentionally skips the SAN/extfile step (reasonable, since clients aren't hostname-verified) — a short clause noting why would avoid confusion given the doc otherwise stresses SAN as required.

Approving as-is.

[esabol]

Good feedback. I'm glad I waited to merge. I've removed -nodes and added notes regarding -CAcreateserial and SAN not being needed for clients/workers.

[esabol]

Merged. The new documentation is visible at https://gearman.org/gearmand/gearmand/ssl/.