Link to contributing and maintainer guides from README

[filmaj]

Summary

Links to the contribution and maintainer guides from the README. Provide more info in maintainer guide how to install Ruby for documentation generation. Remove references to Travis CI.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #1023 (69ee6d2) into main (71ab197) will not change coverage.
The diff coverage is n/a.

❗ Current head 69ee6d2 differs from pull request most recent head 051a4b1. Consider uploading reports for the commit 051a4b1 to get more accurate results

@@           Coverage Diff           @@
##             main    #1023   +/-   ##
=======================================
  Coverage   68.89%   68.89%           
=======================================
  Files          13       13           
  Lines        1212     1212           
  Branches      357      357           
=======================================
  Hits          835      835           
  Misses        304      304           
  Partials       73       73           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 71ab197...051a4b1. Read the comment docs.