chore(docs): update contributing guidelines (charmkeeper)

[erinecon]

This PR updates the contributing guidelines for the HTTP proxy 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 a115bc1

Test coverage for a115bc1

Name           Stmts   Miss Branch BrPart  Cover   Missing
----------------------------------------------------------
src/charm.py      54      9      2      1    82%   72-78, 99-101, 114-116
src/state.py      85      1     16      1    98%   204
----------------------------------------------------------
TOTAL            139     10     18      2    92%

Static code analysis report

Run started:2026-03-26 15:51:28.675483+00:00

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 975
  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 a115bc1

Test coverage for a115bc1

Name            Stmts   Miss Branch BrPart  Cover   Missing
-----------------------------------------------------------
src/charm.py      140     31     36     14    73%   103-106, 111-112, 116, 123-125, 130->136, 135, 138, 149-150, 152, 154->160, 159, 165-166, 173-176, 209-210, 231, 237, 240, 255-278
src/policy.py      98     68     20      0    25%   24-26, 31, 40-44, 55, 72-76, 89-103, 133-134, 142, 175-196, 219-238, 249-257, 265-271, 279-284, 295-298
src/relay.py      142      8     46      2    95%   155->178, 252-268, 398-401
src/timer.py       11      4      0      0    64%   48-61
-----------------------------------------------------------
TOTAL             391    111    102     16    70%

Static code analysis report

Run started:2026-03-26 15:51:47.356468+00:00

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 1810
  Total lines skipped (#nosec): 6
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 1

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 a115bc1

Test coverage for a115bc1

Name           Stmts   Miss Branch BrPart  Cover   Missing
----------------------------------------------------------
src/charm.py     176     36     38      8    78%   67-69, 76-77, 81, 94, 97-98, 128, 170-200, 213->228, 221-227, 353-367
src/squid.py     110      6     34      5    92%   65, 75, 175, 264, 313, 322, 344->351, 346->348
----------------------------------------------------------
TOTAL            286     42     72     13    84%

Static code analysis report

Run started:2026-03-26 15:51:39.660148+00:00

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 2243
  Total lines skipped (#nosec): 1
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 1

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):