Fix defaults handling, perform linting

[sammcj]

This PR is for two things:

1. Currently when trying to use this Action linting/actionlint fails because all optional inputs are marked as required: true.

I have refactored the bash parameters to set the defaults if no input is provided, and set the inputs to optional.

2. Fixed numerous markdown formatting errors.

cc/ @gaurav-nelson

[sammcj]

The two markdown link errors are both false positives @gaurav-nelson

[sammcj]

@gaurav-nelson do you know what's up with those false positive link checks?

[gaurav-nelson]

For GitHub can you see #136 (comment) and add it to the mlc-config file ?

The other one is probably disallowing agents. maybe add it to ignore list as well.

[sammcj]

@gaurav-nelson thanks, done :)

[gaurav-nelson]

@sammcj Can you please check the script again, I think the failed workflow is not running with the --config option. I am not sure though.

Also, See https://github.com/gaurav-nelson/github-action-markdown-link-check/actions/runs/3140346552/jobs/5101650177#step:3:154 which outputs the command we are running however the failed check doesn't list the command https://github.com/gaurav-nelson/github-action-markdown-link-check/actions/runs/3140346552/jobs/5101650203? 🤔

[sammcj]

@gaurav-nelson Oh I see what's going on, it's relying on on all inputs being provided in order to have each parameter in the right place.

OK I'll refactor!