docs: improve landing pages for Contrib Guide

[StackScribe]

Summary

This PR does the following:

• Remove "under development" disclaimer for "General" and "Software" sections of the "Contributing" guide
• Adds brief description of the sub-sections of the "Contributing" guide on the main landing page
• Adds brief description of the "General" section on that landing page
• Adds brief description of the "Software contributions" section on that landing page
• Adds some xrefs and content enhancements to "General" sub-sections

Checks

• [n/a] My PR fulfills the Definition of Done of the corresponding issue and not more (or parts if the issue is separated
  into multiple PRs)
• I used descriptive commit messages to help reviewers understand my thought process
• I signed off all my commits according to the Developer Certificate of Origin (DCO)(
  see Contribution Guide)
• My PR title is formatted according to the semantic PR conventions described in
  the Contribution Guide
• My content follows the style guidelines of this project (YAMLLint, markdown-lint)
• [n/a] I regenerated the auto-generated docs for Helm and the CRD documentation (if applicable)
• I have performed a self-review of my content including grammar and typo errors and also checked the rendered page
  from the Netlify deploy preview
• My changes result in all-green PR checks (first-time contributors need to ask a maintainer to approve their test runs)

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarCloud