Docs: Clarify tasks section

Move simple tasks example into an actual `.sby` file and use `literalinclude`.
Add `gen.sh` script for generating output log (currently using checked in log file so that we don't need to deal with having sby installed for readthedocs).
Remove more complex examples.
Remove "group aliases" in favor of "tags".
Refer to `<tag>:` instead of `<taskname>:`.
Introduce `--dumptags` and tasks being assigned a tag of the same name.
Split out tags into subsection.
Move "run specific task" example call in line, and add a task (so that it shows two tasks in one)

docs/examples/tags/example.log

@@ -0,0 +1,18 @@
+$ sby --dumptasks example.sby
+task1
+task2
+task3
+$ sby --dumptags example.sby
+task1
+task2
+task3
+task_1_or_2
+task_1_or_3
+$ sby --dumptags example.sby task1
+task1
+task_1_or_2
+task_1_or_3
+$ sby --dumpcfg example.sby task1
+[options]
+mode bmc
+depth 100

docs/examples/tags/example.sby

@@ -0,0 +1,9 @@
+[tasks]
+task1 task_1_or_2 task_1_or_3
+task2 task_1_or_2
+task3 task_1_or_3
+
+[options]
+task_1_or_2: mode bmc
+task3: mode prove
+~task3: depth 100

docs/examples/tags/gen.sh

@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+set -eu
+
+sby="sby"
+script="example.sby"
+task="task1"
+
+while getopts :s: opt; do
+    case "$opt" in
+        s)
+            sby=$OPTARG
+            ;;
+        *)
+            echo "Usage: $0 [-s SBY command]\n" >&2
+            exit 1
+            ;;
+    esac
+done
+
+# log our commands and output
+PS4="$ "
+exec > >(tee example.log) 2>&1
+set -x
+
+$sby --dumptasks $script
+$sby --dumptags $script
+$sby --dumptags $script $task
+$sby --dumpcfg $script $task