doc: add a page on starting Flux

[garlick]

This adds a readthedocs page containing several ways to start a flux instance, with more explanation that we've given in our other higher level docs.

[chu11]

began reading this, I began thinking maybe this belongs more in flux-docs?

It might be a good "lead-in" document before some of the more specific tutorials, such as my forever WIP flux-framework/flux-docs#225

[garlick]

Admittedly this section is useful stuff for flux framework generally but my goal (as noted in #5470) was to build up some flux-core specific documentation to complement the manual pages.

[garlick]

I made a couple of changes to the last sections.

I dropped the LSF section since it appears that LSF users normally launch MPI programs with the mpirun for whatever MPI they are using, so a more appropriate section would be about how to start flux with the different mpirun variants. I already have the one from mpich (hydra). For now, I deleted the LSF section and we can leave the specialized jsrun instructions that apply only to LLNL and oak ridge to the coral document we already have.

I changed the "Starting with systemd" section to a more detailed and generalized "Starting with Static Configuration" section and mentioned the operator and system instance in that section with external references.

[codecov]

Codecov Report

Merging #5477 (03d7a16) into master (544f916) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5477      +/-   ##
==========================================
- Coverage   83.65%   83.62%   -0.04%     
==========================================
  Files         483      483              
  Lines       81427    81427              
==========================================
- Hits        68120    68090      -30     
- Misses      13307    13337      +30     

see 12 files with indirect coverage changes

[chu11]

changes LGTM

[garlick]

Thanks @chu11!