Update ycmd and documentation

[Felixoid]

PR Prelude

Thank you for working on YCM! :)

Please complete these steps and check these boxes (by putting an x inside
the brackets) before filing your PR:

• I have read and understood YCM's CONTRIBUTING document.
• I have read and understood YCM's CODE_OF_CONDUCT document.
• I have included tests for the changes in my PR. If not, I have included a
  rationale for why I haven't.
• I understand my PR may be closed if it becomes obvious I didn't
  actually perform all of these steps.

Why this change is necessary and useful

• Update submodule to the most recent state
• Use cross-repository reference to avoid outdated description

Partially depends on ycm-core/ycmd#1811

Here's the link that will be used after the blocking PR is merged https://github.com/Felixoid/ycmd/tree/14e964dda5ff432ceb4cb06fd0ad80a09653f39e#language_server-configuration

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This change is 

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.80%. Comparing base (159e8de) to head (583a623).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4326      +/-   ##
==========================================
+ Coverage   89.67%   89.80%   +0.12%     
==========================================
  Files          37       37              
  Lines        4765     4765              
==========================================
+ Hits         4273     4279       +6     
+ Misses        492      486       -6     

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Felixoid]

Anything blocking it from merging?

cc @puremourning

[Felixoid]

A kind ping? It would drastically improve the quality of life.

[puremourning]

@puremourning reviewed 2 files and all commit messages, and made 1 comment.
Reviewable status: 1 of 2 LGTMs obtained (waiting on @Felixoid).

[puremourning]

Thanks for this, sorry for the delay. I though I had clicked the button but I must have forgotten.