UG update and standardisation

[daitenshionyan]

Changes:

• Added planned features section.
• Added color description of vaccination requirement tags.
• Standardised command short description to sound more like a command.
• Added validation warning for patient and vaccination edit and appointment unmark.
• Made headings mroe prominent.

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: -0.13 :warning:

Comparison is base (f08cce5) 49.60% compared to head (55ffdc7) 49.47%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #366 +/- ## ============================================ - Coverage 49.60% 49.47% -0.13% - Complexity 731 732 +1 ============================================ Files 165 165 Lines 3639 3648 +9 Branches 379 381 +2 ============================================ Hits 1805 1805 - Misses 1755 1762 +7 - Partials 79 81 +2 ``` [see 4 files with indirect coverage changes](https://codecov.io/gh/AY2223S2-CS2103-F11-3/tp/pull/366/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=AY2223S2-CS2103-F11-3) 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-CS2103-F11-3). 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-CS2103-F11-3)

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.