docs: contribution, style: specify that tabs are 8 characters

GitHub, in a moment of infinite wisdom, decided to change the default
tab from eight to four characters.

According to the GitHub announcement the problem that the change
addressed is that "many users are unaware that tab size can be adjusted
in their settings". As a result now users with this so called "improved"
setting sees a different indentation from all other users.

Help GitHub in their awareness mission, by making the users aware that
tab size can (or rather, has to) be adjusted in their settings to the
previous default, by mentioning it in the contribution guideline.

Call it out in the quick style guide too since it's not the default
anymore.

Link: https://github.blog/changelog/2025-08-07-default-tab-size-changed-from-eight-to-four/


Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>