Make the naked invocation display a compacted help

[isidentical]

E.g:

$ http           
usage:
    http [METHOD] URL [REQUEST_ITEM ...]

error:
    the following arguments are required: URL

For more information:
    - Try running http --help
    - Or visiting https://httpie.io/docs/cli

[codecov-commenter]

Codecov Report

Merging #1296 (e4ff88d) into master (4d7d6b6) will decrease coverage by 0.74%.
The diff coverage is 95.12%.

❗ Current head e4ff88d differs from pull request most recent head ab5b47a. Consider uploading reports for the commit ab5b47a to get more accurate results

@@            Coverage Diff             @@
##           master    #1296      +/-   ##
==========================================
- Coverage   97.28%   96.53%   -0.75%     
==========================================
  Files          67       86      +19     
  Lines        4235     6040    +1805     
==========================================
+ Hits         4120     5831    +1711     
- Misses        115      209      +94     

Impacted Files	Coverage Δ
tests/test_binary.py	100.00% <ø> (ø)
httpie/compat.py	31.11% <27.90%> (-68.89%)	⬇️
tests/conftest.py	77.14% <58.33%> (-9.82%)	⬇️
httpie/output/lexers/http.py	59.09% <59.09%> (ø)
tests/test_ssl.py	91.01% <66.66%> (-3.93%)	⬇️
httpie/manager/__main__.py	82.35% <82.35%> (ø)
httpie/output/lexers/metadata.py	82.35% <82.35%> (ø)
httpie/models.py	94.23% <90.00%> (-3.14%)	⬇️
tests/test_uploads.py	97.70% <92.59%> (-2.30%)	⬇️
httpie/manager/core.py	92.85% <92.85%> (ø)
... and 75 more

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[isidentical]

@jakubroztocil as requested, I've separated the docs URL from the --help part, and explicitly mentioned about running http --help.