docs: p2p package overview

[algorandskiy]

Summary

1. Update network/p2p/README.md
2. Add network/README-P2P.md

While writing this doc found and fixed the following:

• Make some methods package-private since there is no clients for them.
• Handle peer.GetHTTPClient() returning nil since MakeHTTPClient for p2p net can err.
• Enable capabilities only if node is configured to listen.

[codecov]

Codecov Report

Attention: Patch coverage is 67.85714% with 9 lines in your changes missing coverage. Please review.

Project coverage is 55.93%. Comparing base (23a04c2) to head (c1fd68b).
Report is 1 commits behind head on master.

Files	Patch %	Lines
catchup/ledgerFetcher.go	50.00%	1 Missing and 1 partial ⚠️
catchup/universalFetcher.go	50.00%	1 Missing and 1 partial ⚠️
network/p2p/p2p.go	0.00%	2 Missing ⚠️
network/p2pPeer.go	75.00%	2 Missing ⚠️
network/p2pNetwork.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6096      +/-   ##
==========================================
- Coverage   56.25%   55.93%   -0.32%     
==========================================
  Files         490      490              
  Lines       69699    69706       +7     
==========================================
- Hits        39206    38993     -213     
- Misses      27825    28004     +179     
- Partials     2668     2709      +41     

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[jasonpaulos]

Looks good I have just a few minor comments

[gmalouf]

Put in some wording suggestions for the READMEs along with a question about the isGossipServer checks.

[algorandskiy]

@jasonpaulos / @gmalouf thank you for corrections! All applied.

[gmalouf]

Overall I like - I do wonder where we put the clear recommendation to bias towards using the appropriate hybrid profiles (and making sure they are aware of the appropriate settings) for users interesting in exercising the P2P network out of the gate.