doc: use a venv for python deps

doc/.gitignore

@@ -1,3 +1,4 @@
+/venv
 book/*
 src/clients/cashierd_jsonrpc.md
 src/clients/darkfid_jsonrpc.md

doc/README.md

@@ -7,6 +7,18 @@ https://darkrenaissance.github.io/darkfi
 When adding or removing a section of the book, make sure to update the
 [SUMMARY.md](src/SUMMARY.md) file to actually list the contents.
 
+Use a python virtual environment to install its requirements:
+```shell
+% python -m venv venv
+% source venv/bin/activate
+```
+
+Then install the requirements:
+
+```shell
+% pip install -r requirements.txt
+```
+
 For the mdbook-katex backend run:
 
 ```

doc/requirements.txt

@@ -0,0 +1 @@
+prettytable