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

script/example-app

@@ -87,7 +87,7 @@ start_container() {
 # start_moved_container: run the app's container with the state files on
 # a new volume at /srv/state, which SWWAF_STATE_DIR_FILE names in a file
 # only the smallwebwaf user can read, as under "Settings given as files"
-# in README.md, and wait until it is healthy. smallwebwaf can write the
+# in docs/settings.md, and wait until it is healthy. smallwebwaf can write the
 # new volume only once the run script has given it to that user.
 start_moved_container() {
     docker volume create "$MOVED_VOLUME" >/dev/null