main will be the default branch for gef-extras and gef installation

[therealdreg]

Before this PR, gef-extras.sh installation script uses dev branch by default.

main will be the default branch for gef-extras and gef installation. Also, it should be reflected in documentation.

Also I updated doc because standard users (non-devs) should use --branch main and devs/early_adopters should use --branch dev

IMO this PR must be merged in main (2022.06 - Upset Constant) as hotfix because installation via gef-extras.sh ONLY installs dev branch and dont works correctly with gef main.

Against which architecture was this tested ?

"Tested" indicates that the PR works and the unit test (i.e. make test) run passes without issue.

• x86-32
• x86-64
• ARM
• AARCH64
• MIPS
• POWERPC
• SPARC
• RISC-V

Checklist

• My PR was done against the dev branch, not main.
• My code follows the code style of this project.
• My change includes a change to the documentation, if required.
• If my change adds new code, adequate tests have been added.
• I have read and agree to the CONTRIBUTING document.

[hugsy]

Minor change, we can safely assume that install scripts will be for non-contributors so no problem to change to main. But not for contributors, we'll receive PR against main which is not desirable.