README: Use GitHub Actions badge instead of dysfunct Travis

[MarijnS95]

Seems the last successful build there was 2 years ago, and the badge now says "Build: No longer available". Switch the badge over to reflect the CI status on GitHub Actions instead.

~Note that there is unfortunately no for-the-badge style available here to match the other badges, which we should perhaps update to match GitHub's style instead?~

Also add a badge linking to the documentation of the main v4l crate, wrap some crate names in backticks, and format the markdown documents to have proper spacing around headings.

[raymanfx]

I‘m fine with changing the badge style. You put in more work than I did lately, so I think it’s fair to leave it up to you to choose.

Feel free to change the other ones as well.

[MarijnS95]

Done, plus some other cleanups.

Future work:

• [x] push tags to this repo for every release;
• [x] add missing link labels for tags listed in CHANGELOG.md #83;
• [x] Set up dependabot configuration to bump dependencies #82.

[MarijnS95]

I ran one of the random autoformatters over this document while editing (by accident) and quite like the empty lines it added around headings. I can revert it if it makes things too verbose though, but it shouldn't have any effect on the rendered output.

[raymanfx]

I'm not opposed to changing the formatting, but in that case we need to at least document the process, settle on a linter all of us use etc. Could you add a small section outlining the steps to run the linter?

[MarijnS95]

I think I'll just revert it in that case. I don't want to have strict (CI) processes and linting for two odd markdown files, when it only is the # heading that look off when they are crammed together with the text. Same for the README where there's some unneeded space before * and a crate name not in backticks, that not something to document IMO.