reorganize "Shared Memory and LWLocks" section of docs

I recently began trying to write documentation for the dynamic shared
memory registry feature [0], and I noticed that the "Shared Memory and
LWLocks" section of the documentation might need some improvement. At
least, I felt that it would be hard to add any new content to this section
without making it very difficult to follow.

Concretely, I am proposing breaking it into two sections: one for shared
memory and one for LWLocks. Furthermore, the LWLocks section will be split
into two: one for requesting locks at server startup and one for requesting
locks after server startup. I intend to also split the shared memory
section into at-startup and after-startup sections if/when the dynamic
shared memory registry feature is committed.

Besides this restructuring, I felt that certain parts of this documentation
could benefit from rephrasing and/or additional detail.

Thoughts?

[0] https://postgr.es/m/20231205034647.GA2705267%40nathanxps13

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Nathan Bossart
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