docs: improve build/install commands (#6962)

Changes:

* Format
* Quote URL
* Use `&&` where applicable
* Use parallel make

Kind of relates to commit 500d8f2d6 ("ci: run make in parallel where
applicable", 2023-08-14) / PR #5960.

README

@@ -24,9 +24,11 @@ Please report all security vulnerabilities to:
 
 Compile and install the mainline version from GitHub:
 
-    git clone https://github.com/netblue30/firejail.git
-    cd firejail
-    ./configure && make && sudo make install-strip
+    git clone 'https://github.com/netblue30/firejail.git' &&
+    cd firejail &&
+    ./configure &&
+    make -j "$(nproc)" &&
+    sudo make install-strip
 
 On Debian/Ubuntu you will need to install git and gcc.
 
@@ -43,7 +45,9 @@ derivatives), install libselinux1-dev (libselinux-devel on Fedora) and use the
 We build our release firejail.tar.xz and firejail.deb packages using the
 following commands:
 
-    make distclean && ./configure && make deb
+    make distclean &&
+    ./configure &&
+    make deb
 
 Maintainer:
 - netblue30 (netblue30@protonmail.com)

README.md

@@ -163,9 +163,11 @@ packages](https://github.com/netblue30/firejail/releases).
 You can clone the source code from this git repository and build manually:
 
 ```sh
-git clone https://github.com/netblue30/firejail.git
-cd firejail
-./configure && make && sudo make install-strip
+git clone 'https://github.com/netblue30/firejail.git' &&
+cd firejail &&
+./configure &&
+make -j "$(nproc)" &&
+sudo make install-strip
 ```
 
 On Debian/Ubuntu you will need to install git and gcc.