Added website docs and mdoc. (#1560)

* Added website docs and mdoc. Removed all warnings

* Updated README and added build to circle ci

* Added how to build documentation, deprecated wiki

* Fix copypasta

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

1 files changed, 56 insertions, 0 deletions

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+---
+layout: docs
+title:  "Muxes and Input Selection"
+section: "chisel3"
+---
+Selecting inputs is very useful in hardware description, and therefore Chisel provides several built-in generic input-selection implementations.
+### Mux
+The first one is `Mux`. This is a 2-input selector. Unlike the `Mux2` example which was presented previously, the built-in `Mux` allows
+the inputs (`in0` and `in1`) to be any datatype as long as they are the same subclass of `Data`.
+
+by using the functional module creation feature presented in the previous section, we can create multi-input selector in a simple way:
+
+```scala
+Mux(c1, a, Mux(c2, b, Mux(..., default)))
+```
+
+### MuxCase
+However, this is not necessary since Chisel also provides the built-in `MuxCase`, which implements that exact feature.
+`MuxCase` is an n-way `Mux`, which can be used as follows:
+
+```scala
+MuxCase(default, Array(c1 -> a, c2 -> b, ...))
+```
+
+Where each selection dependency is represented as a tuple in a Scala
+array [ condition -> selected_input_port ].
+
+### MuxLookup
+Chisel also provides `MuxLookup` which is an n-way indexed multiplexer:
+
+```scala
+MuxLookup(idx, default,
+          Array(0.U -> a, 1.U -> b, ...))
+```
+
+This is the same as a `MuxCase`, where the conditions are all index based selection:
+
+```scala
+MuxCase(default,
+        Array((idx === 0.U) -> a,
+              (idx === 1.U) -> b, ...))
+```
+
+Note that the conditions/cases/selectors (eg. c1, c2) must be in parentheses.
+
+### Mux1H
+Another ```Mux``` utility is ```Mux1H``` that takes a sequence of selectors and values and returns the value associated with the one selector that is set. If zero or multiple selectors are set the behavior is undefined.  For example:
+```scala
+  val hotValue = chisel3.util.oneHotMux(Seq(
+    io.selector(0) -> 2.U,
+    io.selector(1) -> 4.U,
+    io.selector(2) -> 8.U,
+    io.selector(4) -> 11.U,
+  ))
+```
+```oneHotMux``` whenever possible generates *Firrtl* that is readily optimizable as low depth and/or tree.  This optimization is not possible when the values are of type ```FixedPoint``` or an aggregate type that contains ```FixedPoint```s and results instead as a simple ```Mux``` tree.  This behavior could be sub-optimal.  As ```FixedPoint``` is still *experimental* this behavior may change in the future.