docs: add trig and twemproxy plugin readmes (#9801)

plugins/inputs/trig/README.md

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+# Trig Input Plugin
+
+The `trig` plugin is for demonstration purposes and inserts sine and cosine
+
+### Configuration
+
+```toml
+# Inserts sine and cosine waves for demonstration purposes
+[[inputs.trig]]
+  ## Set the amplitude
+  amplitude = 10.0
+```
+
+### Metrics
+
+- trig
+  - fields:
+    - cosine (float)
+    - sine (float)
+
+
+### Example Output
+
+```
+trig,host=MBP15-SWANG.local cosine=10,sine=0 1632338680000000000
+trig,host=MBP15-SWANG.local sine=5.877852522924732,cosine=8.090169943749473 1632338690000000000
+trig,host=MBP15-SWANG.local sine=9.510565162951535,cosine=3.0901699437494745 1632338700000000000
+```

plugins/inputs/twemproxy/README.md

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+# Twemproxy Input Plugin
+
+The `twemproxy` plugin gathers statistics from [Twemproxy](https://github.com/twitter/twemproxy) servers.
+
+
+### Configuration
+
+```toml
+# Read Twemproxy stats data
+[[inputs.twemproxy]]
+  ## Twemproxy stats address and port (no scheme)
+  addr = "localhost:22222"
+  ## Monitor pool name
+  pools = ["redis_pool", "mc_pool"]
+```
+