docs: Fix restructuredtext formatting

Workaround RST not allowing nested inline markup.
Use directives instead.

doc/getting_started.rst

@@ -2,7 +2,8 @@
 Getting Started
 ================
 
-**NOTE: This is a work-in-progress.**
+.. caution::
+    This is a work-in-progress.
 
 The following block of code shows how to use PyStan with a model which studied coaching effects across eight schools (see Section 5.5 of Gelman et al (2003)). This hierarchical model is often called the "eight schools" model.
 

doc/index.rst

@@ -4,7 +4,8 @@ PyStan
 
 Release v\ |version|
 
-**NOTE: This documentation describes a BETA release of PyStan 3.**
+.. caution::
+    This documentation describes a BETA release of PyStan 3.
 
 **PyStan** is a Python interface to Stan, a package for Bayesian inference.
 
@@ -22,10 +23,11 @@ Notable features of PyStan include:
 
 .. _getting-started:
 
-Quickstart
-==========
+Quick start
+===========
 
-**NOTE: Pre-release version must be installed with ``python3 -m pip install --pre pystan``.**
+.. tip::
+    Pre-release versions must be installed with ``python3 -m pip install --pre pystan``.
 
 Install PyStan with ``python3 -m pip install pystan``. (PyStan requires Python 3.7 or higher running on a Linux or macOS system.)
 

doc/installation.rst

@@ -2,7 +2,8 @@
 Installation
 ============
 
-**NOTE: Pre-release version must be installed with ``python3 -m pip install --pre pystan``.**
+.. tip::
+    Pre-release versions must be installed with ``python3 -m pip install --pre pystan``.
 
 In order to install PyStan make sure your system satisfies the following requirements:
 

doc/upgrading.rst

@@ -2,7 +2,8 @@
 Upgrading to Newer Releases
 ===========================
 
-**NOTE: This section is a work-in-progress.**
+.. caution::
+    This section is a work-in-progress.
 
 Upgrading to 3.0
 ================