Revise the Installation part of the Tera Term section of the Serial docs page.

[DotLYHiyou]

I was working on a Micro:bit project that needed the serial tool. I wanted to read the serial docs page to learn how to get serial reading from my computer. Then I noticed, the Tera Term download links didn't work anymore. So after a quick search, I found the actual Tera Term link, so I thought I might just edit it and open a PR with the actual download link, AND some other revisions as well.

Changes In this PR

• Updated download links
• additional info on how to get TO the installer.

I understand if you can't accept this PR for any reason, after all, it is my first PR to a big repository like this.