Auto-generate welcome messages for new contributors and link to guides

[RayStick]

Closes #
References #316

This PR adds welcome messages when a contributor (1) creates their first issue (2) creates their first PR and (3) merges their first PR. These messages make new contributors feel appreciated for their contributions, but can also act as a way of including a link to contributor guides.

Proposed Changes / More Details

• A config.yml file has been added that uses a welcome bot to automatically generate a reply in the thread of an Issue or PR, if it notices that this is the first issue/PR/merge for this user within this repo
• The screenshots below show what these welcome messages look like

❓ Questions:

• Do we like the thank you logo or shall we just keep to text?
• The messages are pretty sparse right now, mainly just linking a new contributor to the contributor docs. The advantage of this is that if contributor guidelines change, we do not need to change this welcome bot message. Also, we currently have a checklist within Issue templates and PRs templates. However, if people want to add anything else to these messages that can be done of course.

---

First Issue

First PR

First merge

---

Change Type

• bugfix (+0.0.1)
• minor (+0.1.0)
• major (+1.0.0)
• refactoring (no version update)
• test (no version update)
• infrastructure (no version update)
• documentation (no version update)
• other

Checklist before review

• I added everything I wanted to add to this PR.
• [Code or tests only] I wrote/updated the necessary docstrings.
• [Code or tests only] I ran and passed tests locally.
• [Documentation only] I built the docs locally.
• My contribution is harmonious with the rest of the code: I'm not introducing repetitions.
• My code respects the adopted style, especially linting conventions.
• The title of this PR is explanatory on its own, enough to be understood as part of a changelog.
• I added or indicated the right labels.

• I added information regarding the timeline of completion for this PR.

• Please, comment on my PR while it's a draft and give me feedback on the development!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.15%. Comparing base (a4d4d69) to head (6de2901).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #476   +/-   ##
=======================================
  Coverage   91.15%   91.15%           
=======================================
  Files           8        8           
  Lines        1006     1006           
=======================================
  Hits          917      917           
  Misses         89       89           

[RayStick]

@smoia and @m-miedema I put you both down as reviewers but please add/replace with others that you think would be good to review this PR, if necessary. Thanks!