Copy docs into Zeek repo directly

This is based on commit 2731def9159247e6da8a3191783c89683363689c from the
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doc/scripts/base/frameworks/sumstats/plugins/index.rst

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+:orphan:
+
+Package: base/frameworks/sumstats/plugins
+=========================================
+
+Plugins for the summary statistics framework.
+
+:doc:`/scripts/base/frameworks/sumstats/plugins/__load__.zeek`
+
+
+:doc:`/scripts/base/frameworks/sumstats/plugins/average.zeek`
+
+   Calculate the average.
+
+:doc:`/scripts/base/frameworks/sumstats/plugins/hll_unique.zeek`
+
+   Calculate the number of unique values (using the HyperLogLog algorithm).
+
+:doc:`/scripts/base/frameworks/sumstats/plugins/last.zeek`
+
+   Keep the last X observations.
+
+:doc:`/scripts/base/frameworks/sumstats/plugins/max.zeek`
+
+   Find the maximum value.
+
+:doc:`/scripts/base/frameworks/sumstats/plugins/min.zeek`
+
+   Find the minimum value.
+
+:doc:`/scripts/base/frameworks/sumstats/plugins/sample.zeek`
+
+   Keep a random sample of values.
+
+:doc:`/scripts/base/frameworks/sumstats/plugins/std-dev.zeek`
+
+   Calculate the standard deviation.
+
+:doc:`/scripts/base/frameworks/sumstats/plugins/variance.zeek`
+
+   Calculate the variance.
+
+:doc:`/scripts/base/frameworks/sumstats/plugins/sum.zeek`
+
+   Calculate the sum.
+
+:doc:`/scripts/base/frameworks/sumstats/plugins/topk.zeek`
+
+   Keep the top-k (i.e., most frequently occurring) observations.
+   
+   This plugin uses a probabilistic algorithm to count the top-k elements.
+   The algorithm (called Space-Saving) is described in the paper Efficient
+   Computation of Frequent and Top-k Elements in Data Streams", by
+   Metwally et al. (2005).
+
+:doc:`/scripts/base/frameworks/sumstats/plugins/unique.zeek`
+
+   Calculate the number of unique values.
+