docs: self-managed comby dev dependency install

[rvantonder]

This updates documentation for installing comby as a dependency. There are constraints that make updating comby automatically in the dev scripts hard. These are, roughly:

• binary distribution for different OSes
• rapid iteration that require faster distribution than available through OS package managers, and
• effort or extra time required when using toolchains and dependencies for building from source

If you are interested in more nitty gritty detail, see the slack thread.

Thus, for development, comby will now be a self-managed install, and treated as an external dependency. It is in fact optional unless you work with automation tooling at this point in time. Nothing changes for CI or deployments. There are many options for installing it so that:

• Developers who aren't reliant on comby for development are unaffected (i.e., no broken dev builds, ever, and this is the priority behind this change.)
• Developers who are reliant on comby but not the most recent versions can use, e.g., brew or grab the binaries from GH and put it on their PATH.
• Developers who are reliant on recent versions with bug fixes or feature extensions can grab it directly from GH if it's not available on brew yet.

Test plan: N/A, that's part of the objective.

[codecov]

Codecov Report

Merging #6902 into master will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6902      +/-   ##
==========================================
+ Coverage   39.06%   39.13%   +0.07%     
==========================================
  Files        1231     1230       -1     
  Lines       63392    63271     -121     
  Branches     6064     6038      -26     
==========================================
- Hits        24765    24763       -2     
+ Misses      36362    36243     -119     
  Partials     2265     2265

Impacted Files	Coverage Δ
cmd/frontend/graphqlbackend/repository.go	20.73% <0%> (-0.93%)	⬇️
web/src/regression/util/codenav.ts	0% <0%> (ø)	⬆️
...src/enterprise/site-admin/SiteAdminLsifJobPage.tsx	0% <0%> (ø)	⬆️
web/src/regression/util/codeintel.ts