There is a command on doc/install page https://golang.org/doc/install that uses an outdated filename that is being shown as an example of how to install go for Linux:

tar -C /usr/local -xzf go1.14.3.linux-amd64.tar.gz

Currently, there is a button that allows downloading .tar file that include go1.15 and its name is different from the command, so that if you follow installation steps you'll get an error that .tar file wasn't found.

Changes purposed:
Added a function that uses a filename from the "Download" button that can be used to update Linux installation command.
This function could be used in the future for other OSes if there will be a need for adding informational data that may change in the future. So there's no need to update such data in the markup files.
If there's something wrong during running JS code (or JS is disabled in the browser) and the filename couldn't be parsed - the old filename is used as an example like before.

Fixes golang/go#41232

Change-Id: Ie389ce73de5393f151ad739b3e97873e1a9be179
GitHub-Last-Rev: 0a93f69393a0b81ee1d869bfb7df612278479c1b
GitHub-Pull-Request: golang/website#15
Reviewed-on: https://go-review.googlesource.com/c/website/+/253118
Run-TryBot: Steve Traut <straut@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Steve Traut <straut@google.com>