feat: add BEAM-ErlangVM Dashboards

[syepes]

Two dashboards for monitoring the BEAM Erlang/Elixir runtime.

[crazyuploader]

Hey @syepes, Thanks for your contribution and for putting these dashboards together.

While testing with a real prometheus_ex setup, several panel metric names didn't match what the library exports. A couple examples:

• erlang_vm_process_count vs erlang_vm_processes
• erlang_mnesia_transaction_commits vs erlang_mnesia_committed_transactions
• beam_memory_binary_bytes and the erlang_vm_allocator_* metrics don't exist under those names at all

Which exporter and version were you running when you built these (prometheus_ex, prometheus_process_collector, something custom)? I want to compare against the same source before I flag this as a mismatch.