Add CSS styling to forc doc (FuelLabs#3471)

This PR adds styling and some updates to the structure of the docs
produced by `forc doc`. It also makes the sidebar visible but the items
aren't added yet so FuelLabs#3170 will remain open until then.

- [x] Figure out where to host CSS, icon & logo files

Closes FuelLabs#3169 

![Screenshot from 2022-11-29
22-01-07](https://user-images.githubusercontent.com/57543709/204707367-77362bd1-9ea5-4b95-8cd2-67c40f57a616.png)
![Screenshot from 2022-11-29
21-53-33](https://user-images.githubusercontent.com/57543709/204707368-1471fac8-2396-410e-b226-89ba9e104eec.png)

There are some TODOs in here that I wouldn't mind some input on, and in
general I always welcome the keen eyes of @kayagokalp

.markdownlintignore

@@ -0,0 +1,3 @@
+# Licenses for `forc doc` assets
+
+forc-plugins/forc-doc/src/licenses/SourceSerif4-LICENSE.md

forc-plugins/forc-doc/Cargo.toml

@@ -15,6 +15,7 @@ comrak = "0.15"
 forc-pkg = { version = "0.31.3", path = "../../forc-pkg" }
 forc-util = { version = "0.31.3", path = "../../forc-util"}
 horrorshow = "0.8.4"
+include_dir = "0.7.3"
 opener = "0.5.0"
 sway-ast = { version = "0.31.3", path = "../../sway-ast" }
 sway-core = { version = "0.31.3", path = "../../sway-core" }