chore(github): Add PR Template

[merkata]

Adds a PR template, seems it is missing for this repo.

[cbartz]

Small comment. Btw, the link to the contribution guide (which explains how-to generate src-docs) is broken. It should point to https://charmhub.io/discourse-k8s/docs/how-to-contribute

[github-actions]

Test coverage for 97d0e49

Name              Stmts   Miss Branch BrPart  Cover   Missing
-------------------------------------------------------------
src/charm.py        325     31     82     11    90%   137, 171, 179-180, 336->341, 533-535, 540-541, 552-554, 559-560, 572-574, 579-580, 592-594, 619-621, 661->exit, 671->exit, 701-707, 733->exit, 747-748, 758->exit, 772
src/database.py      30      1      8      1    95%   56
-------------------------------------------------------------
TOTAL               355     32     90     12    90%

Static code analysis report

Run started:2023-12-11 08:31:53.154807

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 1944
  Total lines skipped (#nosec): 1
  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):