Document all RPC method subdirectories in docs index

[kngako]

The docs/rpc/README.md index only listed a subset of the method categories, leaving several documented method groups undiscoverable:

• Add the missing Analytics category (analytics/README.md): dashboards and analytical reports covering place statistics, daily infrastructure and request-log reports, and top API consumers.
• Add the missing Import category (import/README.md): submission and management of places from trusted external sources, listing the four methods (submit_place, revoke_submitted_place, get_submitted_place, get_place_import_origins) so readers can find the way to add a merchant to the map.
• Point each category that has a subdirectory of per-method docs (auth, event, area, invoice, electrum, wallet) at it and summarize the methods it contains.
• Remove the Log Methods entry: it linked to log/README.md, which does not exist. The log-related methods (get_request_log, get_daily_infra_report) live under analytics/ and are covered by the new Analytics entry.

Every link in the index now resolves to an existing file or directory.