[how-to-docs]: Add steps for installation of completion scripts

[saehejkang]

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

Closes #705

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

I tested all steps locally and developers should be able to just copy/paste the commands.

[jglogan]

@saehejkang Thanks, this is great! A bunch of comments here but they're mainly nits to match the contribution to the style of the rest of the doc. Have a look and follow up in comments if there's anything you think should be done differently.

[jglogan]

@saehejkang Could you rework this commit so that your changes are rebased onto the head of main, instead of merging main into the head of your branch?

Probably something like this on your local copy of your fork:

Get the latest apple/container main:

git checkout main
git fetch upstream
git merge upstream/main

Get rid of the merge commits in your feature branch, and sanity check your log to make sure you see the just your 791e092 and c46212c commits :

git checkout how-to-documentation-for-shell-auto-complete
git reset --hard c46212c
git log

Rebase your changes, and fix any merge conflicts you encounter, and then do a last sanity check to see that your diffs against main make sense:

git rebase main
git diff main

Once you're sure your local commit history looks good, rewrite your remote feature branch with the rebased commits:

git push -f

[saehejkang]

Could you rework this commit so that your changes are rebased onto the head of main, instead of merging main into the head of your branch?

Done!