Update user docs

[Cloud7050]

This PR works towards #127 and #96. It includes a full pass over the landing page & UG, and also edits some other docs.

It tries to account for changes/discussions in #139. It adds a TODO stub for the new filter feature.

• After merging, check the generated docs to ensure there aren't formatting errors.

[codecov]

Codecov Report

Merging #137 (342d5ad) into master (f1604e9) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #137   +/-   ##
=========================================
  Coverage     72.73%   72.73%           
  Complexity      418      418           
=========================================
  Files            75       75           
  Lines          1412     1412           
  Branches        137      137           
=========================================
  Hits           1027     1027           
  Misses          343      343           
  Partials         42       42           

Files	Coverage Δ
...in/java/swe/context/logic/commands/AddCommand.java	100.00% <ø> (ø)
...swe/context/model/util/AlphabeticalComparator.java	71.42% <ø> (ø)
...ava/swe/context/model/util/SampleContactsUtil.java	25.00% <ø> (ø)

[Cloud7050]

I've pushed some formatting fixes to master so I can see it rebuild live.

Looks like keys don't get special formatting.