Update PPP, UG, and DG

[zhacatomn]

Changes

• Updated the PPP with my (Shaun) contributions
• Updated UG with images for the add_elderly, add_volunteer and edit commands
  • I also added a simple img-caption class to caption images if you want to
• Updated DG to include Design Decisions for the edit command. I also shifted the location of the edit_pair example since it was in the wrong place

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: +0.57 :tada:

Comparison is base (eeee40e) 72.83% compared to head (cbbbd98) 73.40%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #273 +/- ## ============================================ + Coverage 72.83% 73.40% +0.57% - Complexity 1109 1114 +5 ============================================ Files 139 139 Lines 3423 3414 -9 Branches 384 383 -1 ============================================ + Hits 2493 2506 +13 + Misses 781 763 -18 + Partials 149 145 -4 ``` [see 12 files with indirect coverage changes](https://codecov.io/gh/AY2223S2-CS2103T-W12-1/tp/pull/273/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=AY2223S2-CS2103T-W12-1) Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=AY2223S2-CS2103T-W12-1). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=AY2223S2-CS2103T-W12-1)

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