Improve documentation on install from build

These are suggestions for improvements on the website:

1. Page Building deno from source:

• reference python-is-python3 linux package to ensure a suffix-less python3 in the PATH;

• merge topics "Building deno" and "Building" to eliminate redundancy and ambiguity;

• create topic "Install from build" with instructions on using cargo install deno --locked;

2. Page "Download and install":

• remove instructions for cargo install ...;

• add link to (new) topic "Install from build".