chore(docs): update contributing guidelines (charmkeeper)

[erinecon]

This PR updates the contributing guidelines for the Content Cache charm.

Changes

• Created docs/changelog.md
• Replaced CONTRIBUTING.md
• Deleted .github/pull_request_template.yaml
• Replaced .github/pull_request_template.md

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This PR was automatically created by charmkeeper.

[github-actions]

Test results for commit 837bffd

Test coverage for 837bffd

Name            Stmts   Miss Branch BrPart  Cover   Missing
-----------------------------------------------------------
src/charm.py       47      0      6      0   100%
src/errors.py       1      0      0      0   100%
src/state.py      139      0     32      0   100%
-----------------------------------------------------------
TOTAL             187      0     38      0   100%

Static code analysis report

Run started:2026-03-26 16:03:57.882222+00:00

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 986
  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):

[github-actions]

Test results for commit 837bffd

Test coverage for 837bffd

Name                   Stmts   Miss Branch BrPart  Cover   Missing
------------------------------------------------------------------
src/certificates.py       21      2      6      2    85%   65-68
src/charm.py             106     12      8      0    89%   64-65, 120, 149, 153-156, 211-213, 223-225
src/errors.py             16      0      0      0   100%
src/nginx_manager.py     199     36     16      4    81%   207, 213-219, 224-226, 229, 277-279, 295-297, 337-342, 417-419, 438-442, 461->463, 543-548
src/state.py             163     13     34      1    92%   154-159, 429-439
src/utilities.py          13      0      0      0   100%
------------------------------------------------------------------
TOTAL                    518     63     64      7    88%

Static code analysis report

Run started:2026-03-26 16:03:30.531430+00:00

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 1208
  Total lines skipped (#nosec): 0
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 2

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):