chore: add markdownlint, markdown link check workflow and with updated markdown file

[xuan-cao-swi]

Description

This PR attempts to address #547 (remove gitter) and #981.

1. A markdown link check, similar to the one used by opentelemetry.io, has been added. This check was run as a sample and identified a few dead links, which have been rectified in this PR.
2. The markdown lint has been configured to ignore the CHANGELOG.md file (e.g., !**/CHANGELOG.md). This is because CHANGELOG.md is believed to be auto-generated based on PR titles. Any issues with this file could potentially cause the entire workflow to fail.
3. The Gitter communication platform has been removed and replaced with a Slack channel. This change is aimed at improving the communication process.

Closes #547
Closes #981

[kaylareopelle]

Thanks, @xuan-cao-swi! Few little things I saw, but this is great as-is. Happy we're getting these issues squared away.

Could you update your PR description to link the issues so that they close when this PR is merged?

The update could look something like:

Closes #547
Closes #981

Here's a doc on how the linking feature works: https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue

[xuan-cao-swi]

Looks like https://github.com/open-telemetry/community/ have a series of change 😕, and I couldn't find the replacement link (e.g. https://github.com/open-telemetry/community/blob/master/CONTRIBUTING.md).

It seems like they just removed some of the link like https://github.com/open-telemetry/community/blob/master/community-membership.md#maintainer, and replace it directly with ruby repo contributing

Link has been fixed.

[kaylareopelle]

Looks like https://github.com/open-telemetry/community/ have a series of change 😕, and I couldn't find the replacement link (e.g. https://github.com/open-telemetry/community/blob/master/CONTRIBUTING.md).

It seems like they just removed some of the link like https://github.com/open-telemetry/community/blob/master/community-membership.md#maintainer, and replace it directly with ruby repo contributing

Link has been fixed.

Strange! I used that community-membership link often. I'll open an issue on the repo to see if we can figure out what's replaced those two files. I think you have a good workaround for now.

[kaylareopelle]

@xuan-cao-swi - One replacement for the community CONTRIBUTING.md file could be this new contributor experience guide: https://github.com/open-telemetry/community/blob/main/guides/contributor/README.md

[kaylareopelle]

@xuan-cao-swi - The membership doc lives here now: https://github.com/open-telemetry/community/blob/main/guides/contributor/membership.md

Could you add it back to the files?

[xuan-cao-swi]

@xuan-cao-swi - The membership doc lives here now: https://github.com/open-telemetry/community/blob/main/guides/contributor/membership.md

Could you add it back to the files?

Yes, I changed the link to https://github.com/open-telemetry/community/blob/main/guides/contributor/membership.md. Sorry for the confusion, I mean I fixed the link in our README for Link has been fixed.