[docs] restructure documentation for LeaderWorkerSet and DisaggregatedSet concepts and overview (#989)

* Updated the overview doc to include both LeaderWorkerSet and DisaggregatedSet apis

Signed-off-by: ahg-g <ahg@google.com>

* Updated the concepts section

Signed-off-by: ahg-g <ahg@google.com>

* Updated overview section

Signed-off-by: ahg-g <ahg@google.com>

* renamed management of LWS section to Operations

Signed-off-by: ahg-g <ahg@google.com>

* move contribution guidelines to the end

Signed-off-by: ahg-g <ahg@google.com>

* split the lws subsections under the concepts page into individual sections

Signed-off-by: ahg-g <ahg@google.com>

* clean up the lws overview page

Signed-off-by: ahg-g <ahg@google.com>

* polish the lws concepts page

Signed-off-by: ahg-g <ahg@google.com>

* updated the pod templates title

Signed-off-by: ahg-g <ahg@google.com>

* removed repeated section from DS concepts page

Signed-off-by: ahg-g <ahg@google.com>

* polish the concepts page

Signed-off-by: ahg-g <ahg@google.com>

* added disaggregatedSet concepts pages

Signed-off-by: ahg-g <ahg@google.com>

* polish

Signed-off-by: ahg-g <ahg@google.com>

* Apply suggestions from code review

Apply copilot suggestions

Signed-off-by: ahg-g <ahg@google.com>

* polish subgroup and topology placement docs

Signed-off-by: ahg-g <ahg@google.com>

* Polish the failure handling section

Signed-off-by: ahg-g <ahg@google.com>

* polish the notes

Signed-off-by: ahg-g <ahg@google.com>

* polish the main concepts landing page

Signed-off-by: ahg-g <ahg@google.com>

* Addressed comments

Signed-off-by: ahg-g <ahg@google.com>

---------

Signed-off-by: ahg-g <ahg@google.com>