README rewritten from scratch so a newcomer can follow it (closes #154, closes #155)

The old README opened with project history and one list of a hundred
settings. The new one leads with what smallwebwaf is, what it protects
against, how to build and deploy it, what happens to a request, the
settings most deployments touch, how to check it works, the entrypoints,
the rationale and the design, as REPO_POLICIES.md requires. The reference
lists, every setting, the rule file format, the endpoints, the request
log's fields, the alert events, the metrics, the state files, the image
and the gitea notes, move to files under docs/, each linked from the
README, with SPEC.md as the one place for behaviour. Every statement was
checked against the code on next.

Model: fable-5-1

docs/metrics.md

@@ -0,0 +1,83 @@
+# The metrics
+
+`GET /_smallwebwaf/metrics` answers in the Prometheus text format. The
+requirements are under "Metrics endpoint" in [`SPEC.md`](../SPEC.md); this file
+lists the metrics. Every metric carries the label `instance`, set to
+`SWWAF_INSTANCE_NAME`; Prometheus keeps it as `exported_instance` unless the
+scrape sets `honor_labels: true`. No metric carries a client's address.
+
+## Traffic
+
+- `smallwebwaf_requests_total`, `smallwebwaf_request_bytes_total` and
+  `smallwebwaf_response_bytes_total`, by `status_class` such as `2xx` and by
+  `action`;
+- `smallwebwaf_request_duration_seconds` and
+  `smallwebwaf_upstream_duration_seconds`, as histograms;
+- `smallwebwaf_requests_in_flight`.
+
+## Limits and bans
+
+- `smallwebwaf_rate_limit_hits_total`, by `window` and by `kind` (`requests`,
+  `bytes` or `refusals`);
+- `smallwebwaf_size_and_time_limit_hits_total`, by `limit`, the setting passed;
+- `smallwebwaf_offences_total`, by `kind`;
+- `smallwebwaf_bans_made_total`, by `cause`;
+- `smallwebwaf_active_bans` and `smallwebwaf_permanent_bans`.
+
+## Attack detection
+
+- `smallwebwaf_rule_matches_total`, by `rule_id` and `action`, and
+  `smallwebwaf_rules_loaded`;
+- `smallwebwaf_waf_matches_total`, by `mode` and `rule_id`, for the rules that
+  fired.
+
+## Countries and AS numbers
+
+For the `SWWAF_METRICS_TOP_N` busiest of each, the rest as `other`:
+
+- `smallwebwaf_country_requests_total`,
+  `smallwebwaf_country_request_bytes_total`,
+  `smallwebwaf_country_response_bytes_total` and
+  `smallwebwaf_country_list_refusals_total`, by `country`;
+- `smallwebwaf_asn_requests_total`, `smallwebwaf_asn_request_bytes_total` and
+  `smallwebwaf_asn_response_bytes_total`, by `asn`.
+
+## Lookups
+
+- `smallwebwaf_geojs_requests_total` and `smallwebwaf_geojs_failures_total`;
+- `smallwebwaf_geojs_unanswered_total`: the requests that needed an answer and
+  went on without it;
+- with the file source,
+  `smallwebwaf_lookup_database_last_read_timestamp_seconds` and
+  `smallwebwaf_lookup_database_read_failures_total`.
+
+## Reputation sources
+
+By `source`: each list's URL, each zone, and `abuseipdb`:
+
+- `smallwebwaf_reputation_hits_total`, `smallwebwaf_reputation_queries_total`
+  and `smallwebwaf_reputation_failures_total`;
+- `smallwebwaf_reputation_last_fetch_timestamp_seconds`, for a list;
+- `smallwebwaf_reputation_daily_budget_remaining`, for AbuseIPDB.
+
+## State files
+
+By `file`:
+
+- `smallwebwaf_state_file_writes_total`,
+  `smallwebwaf_state_file_write_failures_total`,
+  `smallwebwaf_state_file_last_write_timestamp_seconds` and
+  `smallwebwaf_state_file_size_bytes`;
+- `smallwebwaf_state_file_edits_taken_in_total` and
+  `smallwebwaf_state_file_edits_set_aside_total`;
+- `smallwebwaf_tracked_clients`: the clients in the table.
+
+## Remote log, alerts and the process
+
+- while a syslog server is set, `smallwebwaf_remote_log_lines_sent_total`,
+  `smallwebwaf_remote_log_lines_dropped_total` and
+  `smallwebwaf_remote_log_buffer_depth`;
+- by `destination`, `smallwebwaf_alerts_sent_total`,
+  `smallwebwaf_alerts_failed_total`, `smallwebwaf_alerts_suppressed_total` and
+  `smallwebwaf_alerts_dropped_total`;
+- Go's own `go_` metrics and the process's `process_` metrics.