Update contributing guide

[erinecon]

Overview

Update CONTRIBUTING.md and add a template "How to contribute" page to point to CONTRIBUTING.md.
Also, small changes to the placeholder variables in the index.md page.

Rationale

Information for contributors is currently handled in two places for IS charms: Within the CONTRIBUTING.md file on GitHub and in a "How to contribute" page on Charmhub. While it's useful to have information in both places, it doesn't make sense to duplicate the information.

Proposed changes:

• Moved some of the "How to contribute" material into the CONTRIBUTING.md file so the information is condensed.
• Added placeholder variables for the charm name into the files.
• Added a template page "How to contribute" that points to the CONTRIBUTING.md file.

Also in this PR: Changes to index.md proposed in a previous PR.

Juju Events Changes

None

Module Changes

None

Library Changes

None

Checklist

• The charm style guide was applied
• The contributing guide was applied
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation is generated using src-docs
• The documentation for charmhub is updated
• The PR is tagged with appropriate label (urgent, trivial, complex)

[github-actions]

Test coverage for 1a86d74

Name           Stmts   Miss Branch BrPart  Cover   Missing
----------------------------------------------------------
src/charm.py      31      0      4      0   100%
----------------------------------------------------------
TOTAL             31      0      4      0   100%

Static code analysis report

Run started:2024-09-26 13:42:14.188101

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 151
  Total lines skipped (#nosec): 0
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 0

Run metrics:
  Total issues (by severity):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
  Total issues (by confidence):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
Files skipped (0):