docs: add nxcdb dedicated reference page with full command documentation

[m4sk0ff]

Summary

Adds a dedicated getting-started/nxcdb.md reference page for the nxcdb
CLI. The existing database-general-usage.md covers the basics well but
leaves most commands completely undocumented. This PR fills that gap.

What's documented for the first time

• hosts — full table output, all subcommands (dc, spooler,
  zerologon, petitpotam), single host drill-down vs multi-host
  fuzzy match behaviour
• creds — full table output, all subcommands (plaintext, hash,
  add, remove), single credential drill-down showing group
  memberships and admin access
• shares — full table output, single share drill-down showing share
  info, host location, and users with read access
• groups — full table output with real AD group data
• dpapi — all subcommands (browser, chrome, msedge,
  credentials, iex, firefox), real msedge output example
• wcc — full column selection syntax documented
• import and clear_database — both previously undocumented
• Protocol support note — clarifies ftp, rdp, ldap, ssh are
  currently unimplemented in nxcdb

Bug documented

export keys crashes with AttributeError: 'database' object has no attribute 'get_keys' on Python 3.13. Documented in the page with a
danger hint and workaround. Bug report filed separately at Pennyw0rth/NetExec#1278.

Correction to existing docs

database-general-usage.md shows the active workspace marker as ==>
but the actual output uses *. Corrected in this PR.

How the content was verified

All commands and output were tested hands-on in a live lab environment
against real Windows targets. Tables are rendered from actual
session output, not guessed from source code.

Files changed

• getting-started/nxcdb.md — new file
• getting-started/database-general-usage.md — one-line fix to
  workspace list marker