docs: Improve Doxygen group descriptions from remote docs

[kirre-bylund]

Summary

Read each feature's docs.lootlocker.com page and rewrote the @brief and expanded descriptions in .doxygen/groups.dox so they accurately reflect what each system actually does, its key concepts, and its relationship to other groups.

What changed

Every group that was previously a one-liner or a thin restatement of the group name now has a meaningful 3–6 line description. Specific improvements per group:

Group	What improved
MultiUserManagement	Added local-multiplayer use-case context and the three player lifecycle states (active / inactive-cached / cached)
Authentication	Restructured into the three login families (Guest / Platform / White Label) with concrete platform list
WhiteLabel	Added branding customization and Guest→White Label upgrade path
RemoteSessions	Clarified the actual cross-platform scenario (e.g. Switch player authing as Steam) and the auth.game flow
ConnectedAccounts	Distinguished External vs Direct Linking; clarified cross-platform profile persistence
Presence	Clarified Basic (CCU/session length) vs Rich Presence (activity timelines); corrected "online/offline" framing
Friends	Added supported identifier types and cross-platform framing
Followers	Clarified one-way/no-approval distinction vs Friends
Player	Added cross-references to the complementary groups (AssetInstance, Progressions, Balances, Files)
PlayerFiles	Added CDN framing, privacy flag, revision/revert support
PlayerProgressions	Added tier-threshold/reward unlock structure; cross-ref to Progressions
Hero	Added Base Class → Hero Class inheritance explanation
CharacterProgressions	Described per-class/per-hero use cases
Currency	Added denomination chain, distribution via Progressions/Leaderboards/Triggers, cross-ref to Balances
Balances	Added Wallet concept, character wallets, direct credit/debit
Catalog	Added multi-catalog model, bundle support, platform payment system integration
Purchasing	Distinguished the two purchase flows and their entitlement output
Entitlements	Added purchase-history / First Party compliance angle
DropTables	Added drop groups, weighting, Force Select, and tag-based context filtering
Assets	Added Context/Complexity model and reference-vs-instance distinction
AssetInstance	Added instance lifecycle and ingress paths (rewards, purchases, loadouts)
Progressions	Added multi-scope (player/character/asset) framing; cross-refs to all three scoped groups
Leaderboard	Added Player Type vs Generic Type; periodic reset with reward distribution
Triggers	Distinguished Limited vs Unlimited trigger types
Metadata	Added value types, Tags, and Game Read access control
Notifications	Clarified delivery-queue model and that these are NOT mobile push notifications
Broadcasts	Added segment targeting, localization, and scheduling framing
Feedback	Broke out the three feedback targets (Player / UGC / Game)

Closes part of lootlocker/index#1461 (improving documentation quality).